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

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diff --git a/third_party/git/Documentation/technical/api-submodule-config.txt b/third_party/git/Documentation/technical/api-submodule-config.txt
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+submodule config cache API
+==========================
+
+The submodule config cache API allows to read submodule
+configurations/information from specified revisions. Internally
+information is lazily read into a cache that is used to avoid
+unnecessary parsing of the same .gitmodules files. Lookups can be done by
+submodule path or name.
+
+Usage
+-----
+
+To initialize the cache with configurations from the worktree the caller
+typically first calls `gitmodules_config()` to read values from the
+worktree .gitmodules and then to overlay the local git config values
+`parse_submodule_config_option()` from the config parsing
+infrastructure.
+
+The caller can look up information about submodules by using the
+`submodule_from_path()` or `submodule_from_name()` functions. They return
+a `struct submodule` which contains the values. The API automatically
+initializes and allocates the needed infrastructure on-demand. If the
+caller does only want to lookup values from revisions the initialization
+can be skipped.
+
+If the internal cache might grow too big or when the caller is done with
+the API, all internally cached values can be freed with submodule_free().
+
+Data Structures
+---------------
+
+`struct submodule`::
+
+	This structure is used to return the information about one
+	submodule for a certain revision. It is returned by the lookup
+	functions.
+
+Functions
+---------
+
+`void submodule_free(struct repository *r)`::
+
+	Use these to free the internally cached values.
+
+`int parse_submodule_config_option(const char *var, const char *value)`::
+
+	Can be passed to the config parsing infrastructure to parse
+	local (worktree) submodule configurations.
+
+`const struct submodule *submodule_from_path(const unsigned char *treeish_name, const char *path)`::
+
+	Given a tree-ish in the superproject and a path, return the
+	submodule that is bound at the path in the named tree.
+
+`const struct submodule *submodule_from_name(const unsigned char *treeish_name, const char *name)`::
+
+	The same as above but lookup by name.
+
+Whenever a submodule configuration is parsed in `parse_submodule_config_option`
+via e.g. `gitmodules_config()`, it will overwrite the null_sha1 entry.
+So in the normal case, when HEAD:.gitmodules is parsed first and then overlayed
+with the repository configuration, the null_sha1 entry contains the local
+configuration of a submodule (e.g. consolidated values from local git
+configuration and the .gitmodules file in the worktree).
+
+For an example usage see test-submodule-config.c.