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

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+tree walking API
+================
+
+The tree walking API is used to traverse and inspect trees.
+
+Data Structures
+---------------
+
+`struct name_entry`::
+
+	An entry in a tree. Each entry has a sha1 identifier, pathname, and
+	mode.
+
+`struct tree_desc`::
+
+	A semi-opaque data structure used to maintain the current state of the
+	walk.
++
+* `buffer` is a pointer into the memory representation of the tree. It always
+points at the current entry being visited.
+
+* `size` counts the number of bytes left in the `buffer`.
+
+* `entry` points to the current entry being visited.
+
+`struct traverse_info`::
+
+	A structure used to maintain the state of a traversal.
++
+* `prev` points to the traverse_info which was used to descend into the
+current tree. If this is the top-level tree `prev` will point to
+a dummy traverse_info.
+
+* `name` is the entry for the current tree (if the tree is a subtree).
+
+* `pathlen` is the length of the full path for the current tree.
+
+* `conflicts` can be used by callbacks to maintain directory-file conflicts.
+
+* `fn` is a callback called for each entry in the tree. See Traversing for more
+information.
+
+* `data` can be anything the `fn` callback would want to use.
+
+* `show_all_errors` tells whether to stop at the first error or not.
+
+Initializing
+------------
+
+`init_tree_desc`::
+
+	Initialize a `tree_desc` and decode its first entry. The buffer and
+	size parameters are assumed to be the same as the buffer and size
+	members of `struct tree`.
+
+`fill_tree_descriptor`::
+
+	Initialize a `tree_desc` and decode its first entry given the
+	object ID of a tree. Returns the `buffer` member if the latter
+	is a valid tree identifier and NULL otherwise.
+
+`setup_traverse_info`::
+
+	Initialize a `traverse_info` given the pathname of the tree to start
+	traversing from. The `base` argument is assumed to be the `path`
+	member of the `name_entry` being recursed into unless the tree is a
+	top-level tree in which case the empty string ("") is used.
+
+Walking
+-------
+
+`tree_entry`::
+
+	Visit the next entry in a tree. Returns 1 when there are more entries
+	left to visit and 0 when all entries have been visited. This is
+	commonly used in the test of a while loop.
+
+`tree_entry_len`::
+
+	Calculate the length of a tree entry's pathname. This utilizes the
+	memory structure of a tree entry to avoid the overhead of using a
+	generic strlen().
+
+`update_tree_entry`::
+
+	Walk to the next entry in a tree. This is commonly used in conjunction
+	with `tree_entry_extract` to inspect the current entry.
+
+`tree_entry_extract`::
+
+	Decode the entry currently being visited (the one pointed to by
+	`tree_desc's` `entry` member) and return the sha1 of the entry. The
+	`pathp` and `modep` arguments are set to the entry's pathname and mode
+	respectively.
+
+`get_tree_entry`::
+
+	Find an entry in a tree given a pathname and the sha1 of a tree to
+	search. Returns 0 if the entry is found and -1 otherwise. The third
+	and fourth parameters are set to the entry's sha1 and mode
+	respectively.
+
+Traversing
+----------
+
+`traverse_trees`::
+
+	Traverse `n` number of trees in parallel. The `fn` callback member of
+	`traverse_info` is called once for each tree entry.
+
+`traverse_callback_t`::
+	The arguments passed to the traverse callback are as follows:
++
+* `n` counts the number of trees being traversed.
+
+* `mask` has its nth bit set if something exists in the nth entry.
+
+* `dirmask` has its nth bit set if the nth tree's entry is a directory.
+
+* `entry` is an array of size `n` where the nth entry is from the nth tree.
+
+* `info` maintains the state of the traversal.
+
++
+Returning a negative value will terminate the traversal. Otherwise the
+return value is treated as an update mask. If the nth bit is set the nth tree
+will be updated and if the bit is not set the nth tree entry will be the
+same in the next callback invocation.
+
+`make_traverse_path`::
+
+	Generate the full pathname of a tree entry based from the root of the
+	traversal. For example, if the traversal has recursed into another
+	tree named "bar" the pathname of an entry "baz" in the "bar"
+	tree would be "bar/baz".
+
+`traverse_path_len`::
+
+	Calculate the length of a pathname returned by `make_traverse_path`.
+	This utilizes the memory structure of a tree entry to avoid the
+	overhead of using a generic strlen().
+
+Authors
+-------
+
+Written by Junio C Hamano <gitster@pobox.com> and Linus Torvalds
+<torvalds@linux-foundation.org>