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

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+git-update-ref(1)
+=================
+
+NAME
+----
+git-update-ref - Update the object name stored in a ref safely
+
+SYNOPSIS
+--------
+[verse]
+'git update-ref' [-m <reason>] [--no-deref] (-d <ref> [<oldvalue>] | [--create-reflog] <ref> <newvalue> [<oldvalue>] | --stdin [-z])
+
+DESCRIPTION
+-----------
+Given two arguments, stores the <newvalue> in the <ref>, possibly
+dereferencing the symbolic refs.  E.g. `git update-ref HEAD
+<newvalue>` updates the current branch head to the new object.
+
+Given three arguments, stores the <newvalue> in the <ref>,
+possibly dereferencing the symbolic refs, after verifying that
+the current value of the <ref> matches <oldvalue>.
+E.g. `git update-ref refs/heads/master <newvalue> <oldvalue>`
+updates the master branch head to <newvalue> only if its current
+value is <oldvalue>.  You can specify 40 "0" or an empty string
+as <oldvalue> to make sure that the ref you are creating does
+not exist.
+
+It also allows a "ref" file to be a symbolic pointer to another
+ref file by starting with the four-byte header sequence of
+"ref:".
+
+More importantly, it allows the update of a ref file to follow
+these symbolic pointers, whether they are symlinks or these
+"regular file symbolic refs".  It follows *real* symlinks only
+if they start with "refs/": otherwise it will just try to read
+them and update them as a regular file (i.e. it will allow the
+filesystem to follow them, but will overwrite such a symlink to
+somewhere else with a regular filename).
+
+If --no-deref is given, <ref> itself is overwritten, rather than
+the result of following the symbolic pointers.
+
+In general, using
+
+	git update-ref HEAD "$head"
+
+should be a _lot_ safer than doing
+
+	echo "$head" > "$GIT_DIR/HEAD"
+
+both from a symlink following standpoint *and* an error checking
+standpoint.  The "refs/" rule for symlinks means that symlinks
+that point to "outside" the tree are safe: they'll be followed
+for reading but not for writing (so we'll never write through a
+ref symlink to some other tree, if you have copied a whole
+archive by creating a symlink tree).
+
+With `-d` flag, it deletes the named <ref> after verifying it
+still contains <oldvalue>.
+
+With `--stdin`, update-ref reads instructions from standard input and
+performs all modifications together.  Specify commands of the form:
+
+	update SP <ref> SP <newvalue> [SP <oldvalue>] LF
+	create SP <ref> SP <newvalue> LF
+	delete SP <ref> [SP <oldvalue>] LF
+	verify SP <ref> [SP <oldvalue>] LF
+	option SP <opt> LF
+
+With `--create-reflog`, update-ref will create a reflog for each ref
+even if one would not ordinarily be created.
+
+Quote fields containing whitespace as if they were strings in C source
+code; i.e., surrounded by double-quotes and with backslash escapes.
+Use 40 "0" characters or the empty string to specify a zero value.  To
+specify a missing value, omit the value and its preceding SP entirely.
+
+Alternatively, use `-z` to specify in NUL-terminated format, without
+quoting:
+
+	update SP <ref> NUL <newvalue> NUL [<oldvalue>] NUL
+	create SP <ref> NUL <newvalue> NUL
+	delete SP <ref> NUL [<oldvalue>] NUL
+	verify SP <ref> NUL [<oldvalue>] NUL
+	option SP <opt> NUL
+
+In this format, use 40 "0" to specify a zero value, and use the empty
+string to specify a missing value.
+
+In either format, values can be specified in any form that Git
+recognizes as an object name.  Commands in any other format or a
+repeated <ref> produce an error.  Command meanings are:
+
+update::
+	Set <ref> to <newvalue> after verifying <oldvalue>, if given.
+	Specify a zero <newvalue> to ensure the ref does not exist
+	after the update and/or a zero <oldvalue> to make sure the
+	ref does not exist before the update.
+
+create::
+	Create <ref> with <newvalue> after verifying it does not
+	exist.  The given <newvalue> may not be zero.
+
+delete::
+	Delete <ref> after verifying it exists with <oldvalue>, if
+	given.  If given, <oldvalue> may not be zero.
+
+verify::
+	Verify <ref> against <oldvalue> but do not change it.  If
+	<oldvalue> zero or missing, the ref must not exist.
+
+option::
+	Modify behavior of the next command naming a <ref>.
+	The only valid option is `no-deref` to avoid dereferencing
+	a symbolic ref.
+
+If all <ref>s can be locked with matching <oldvalue>s
+simultaneously, all modifications are performed.  Otherwise, no
+modifications are performed.  Note that while each individual
+<ref> is updated or deleted atomically, a concurrent reader may
+still see a subset of the modifications.
+
+LOGGING UPDATES
+---------------
+If config parameter "core.logAllRefUpdates" is true and the ref is one under
+"refs/heads/", "refs/remotes/", "refs/notes/", or the symbolic ref HEAD; or
+the file "$GIT_DIR/logs/<ref>" exists then `git update-ref` will append
+a line to the log file "$GIT_DIR/logs/<ref>" (dereferencing all
+symbolic refs before creating the log name) describing the change
+in ref value.  Log lines are formatted as:
+
+    oldsha1 SP newsha1 SP committer LF
+
+Where "oldsha1" is the 40 character hexadecimal value previously
+stored in <ref>, "newsha1" is the 40 character hexadecimal value of
+<newvalue> and "committer" is the committer's name, email address
+and date in the standard Git committer ident format.
+
+Optionally with -m:
+
+    oldsha1 SP newsha1 SP committer TAB message LF
+
+Where all fields are as described above and "message" is the
+value supplied to the -m option.
+
+An update will fail (without changing <ref>) if the current user is
+unable to create a new log file, append to the existing log file
+or does not have committer information available.
+
+GIT
+---
+Part of the linkgit:git[1] suite