merge(third_party/git): Merge squashed git subtree at v2.23.0 r/373

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+From:	Junio C Hamano <gitster@pobox.com>
+To:	git@vger.kernel.org
+Cc:	Petr Baudis <pasky@suse.cz>, Linus Torvalds <torvalds@osdl.org>
+Subject: Re: sending changesets from the middle of a git tree
+Date:	Sun, 14 Aug 2005 18:37:39 -0700
+Abstract: In this article, JC talks about how he rebases the
+ public "pu" branch using the core Git tools when he updates
+ the "master" branch, and how "rebase" works.  Also discussed
+ is how this applies to individual developers who sends patches
+ upstream.
+Content-type: text/asciidoc
+
+How to rebase from an internal branch
+=====================================
+
+--------------------------------------
+Petr Baudis <pasky@suse.cz> writes:
+
+> Dear diary, on Sun, Aug 14, 2005 at 09:57:13AM CEST, I got a letter
+> where Junio C Hamano <junkio@cox.net> told me that...
+>> Linus Torvalds <torvalds@osdl.org> writes:
+>>
+>> > Junio, maybe you want to talk about how you move patches from your "pu"
+>> > branch to the real branches.
+>>
+> Actually, wouldn't this be also precisely for what StGIT is intended to?
+--------------------------------------
+
+Exactly my feeling.  I was sort of waiting for Catalin to speak
+up.  With its basing philosophical ancestry on quilt, this is
+the kind of task StGIT is designed to do.
+
+I just have done a simpler one, this time using only the core
+Git tools.
+
+I had a handful of commits that were ahead of master in pu, and I
+wanted to add some documentation bypassing my usual habit of
+placing new things in pu first.  At the beginning, the commit
+ancestry graph looked like this:
+
+                             *"pu" head
+    master --> #1 --> #2 --> #3
+
+So I started from master, made a bunch of edits, and committed:
+
+    $ git checkout master
+    $ cd Documentation; ed git.txt ...
+    $ cd ..; git add Documentation/*.txt
+    $ git commit -s
+
+After the commit, the ancestry graph would look like this:
+
+                              *"pu" head
+    master^ --> #1 --> #2 --> #3
+          \
+            \---> master
+
+The old master is now master^ (the first parent of the master).
+The new master commit holds my documentation updates.
+
+Now I have to deal with "pu" branch.
+
+This is the kind of situation I used to have all the time when
+Linus was the maintainer and I was a contributor, when you look
+at "master" branch being the "maintainer" branch, and "pu"
+branch being the "contributor" branch.  Your work started at the
+tip of the "maintainer" branch some time ago, you made a lot of
+progress in the meantime, and now the maintainer branch has some
+other commits you do not have yet.  And "git rebase" was written
+with the explicit purpose of helping to maintain branches like
+"pu".  You _could_ merge master to pu and keep going, but if you
+eventually want to cherrypick and merge some but not necessarily
+all changes back to the master branch, it often makes later
+operations for _you_ easier if you rebase (i.e. carry forward
+your changes) "pu" rather than merge.  So I ran "git rebase":
+
+    $ git checkout pu
+    $ git rebase master pu
+
+What this does is to pick all the commits since the current
+branch (note that I now am on "pu" branch) forked from the
+master branch, and forward port these changes.
+
+    master^ --> #1 --> #2 --> #3
+          \                                  *"pu" head
+            \---> master --> #1' --> #2' --> #3'
+
+The diff between master^ and #1 is applied to master and
+committed to create #1' commit with the commit information (log,
+author and date) taken from commit #1.  On top of that #2' and #3'
+commits are made similarly out of #2 and #3 commits.
+
+Old #3 is not recorded in any of the .git/refs/heads/ file
+anymore, so after doing this you will have dangling commit if
+you ran fsck-cache, which is normal.  After testing "pu", you
+can run "git prune" to get rid of those original three commits.
+
+While I am talking about "git rebase", I should talk about how
+to do cherrypicking using only the core Git tools.
+
+Let's go back to the earlier picture, with different labels.
+
+You, as an individual developer, cloned upstream repository and
+made a couple of commits on top of it.
+
+                              *your "master" head
+   upstream --> #1 --> #2 --> #3
+
+You would want changes #2 and #3 incorporated in the upstream,
+while you feel that #1 may need further improvements.  So you
+prepare #2 and #3 for e-mail submission.
+
+    $ git format-patch master^^ master
+
+This creates two files, 0001-XXXX.patch and 0002-XXXX.patch.  Send
+them out "To: " your project maintainer and "Cc: " your mailing
+list.  You could use contributed script git-send-email if
+your host has necessary perl modules for this, but your usual
+MUA would do as long as it does not corrupt whitespaces in the
+patch.
+
+Then you would wait, and you find out that the upstream picked
+up your changes, along with other changes.
+
+   where                      *your "master" head
+  upstream --> #1 --> #2 --> #3
+    used   \
+   to be     \--> #A --> #2' --> #3' --> #B --> #C
+                                                *upstream head
+
+The two commits #2' and #3' in the above picture record the same
+changes your e-mail submission for #2 and #3 contained, but
+probably with the new sign-off line added by the upstream
+maintainer and definitely with different committer and ancestry
+information, they are different objects from #2 and #3 commits.
+
+You fetch from upstream, but not merge.
+
+    $ git fetch upstream
+
+This leaves the updated upstream head in .git/FETCH_HEAD but
+does not touch your .git/HEAD or .git/refs/heads/master.
+You run "git rebase" now.
+
+    $ git rebase FETCH_HEAD master
+
+Earlier, I said that rebase applies all the commits from your
+branch on top of the upstream head.  Well, I lied.  "git rebase"
+is a bit smarter than that and notices that #2 and #3 need not
+be applied, so it only applies #1.  The commit ancestry graph
+becomes something like this:
+
+   where                     *your old "master" head
+  upstream --> #1 --> #2 --> #3
+    used   \                      your new "master" head*
+   to be     \--> #A --> #2' --> #3' --> #B --> #C --> #1'
+                                                *upstream
+                                                head
+
+Again, "git prune" would discard the disused commits #1-#3 and
+you continue on starting from the new "master" head, which is
+the #1' commit.
+
+-jc