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

Merge commit '1b593e1ea4d2af0f6444d9a7788d5d99abd6fde5' as 'third_party/git'

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+gitattributes API
+=================
+
+gitattributes mechanism gives a uniform way to associate various
+attributes to set of paths.
+
+
+Data Structure
+--------------
+
+`struct git_attr`::
+
+	An attribute is an opaque object that is identified by its name.
+	Pass the name to `git_attr()` function to obtain the object of
+	this type.  The internal representation of this structure is
+	of no interest to the calling programs.  The name of the
+	attribute can be retrieved by calling `git_attr_name()`.
+
+`struct attr_check_item`::
+
+	This structure represents one attribute and its value.
+
+`struct attr_check`::
+
+	This structure represents a collection of `attr_check_item`.
+	It is passed to `git_check_attr()` function, specifying the
+	attributes to check, and receives their values.
+
+
+Attribute Values
+----------------
+
+An attribute for a path can be in one of four states: Set, Unset,
+Unspecified or set to a string, and `.value` member of `struct
+attr_check_item` records it.  There are three macros to check these:
+
+`ATTR_TRUE()`::
+
+	Returns true if the attribute is Set for the path.
+
+`ATTR_FALSE()`::
+
+	Returns true if the attribute is Unset for the path.
+
+`ATTR_UNSET()`::
+
+	Returns true if the attribute is Unspecified for the path.
+
+If none of the above returns true, `.value` member points at a string
+value of the attribute for the path.
+
+
+Querying Specific Attributes
+----------------------------
+
+* Prepare `struct attr_check` using attr_check_initl()
+  function, enumerating the names of attributes whose values you are
+  interested in, terminated with a NULL pointer.  Alternatively, an
+  empty `struct attr_check` can be prepared by calling
+  `attr_check_alloc()` function and then attributes you want to
+  ask about can be added to it with `attr_check_append()`
+  function.
+
+* Call `git_check_attr()` to check the attributes for the path.
+
+* Inspect `attr_check` structure to see how each of the
+  attribute in the array is defined for the path.
+
+
+Example
+-------
+
+To see how attributes "crlf" and "ident" are set for different paths.
+
+. Prepare a `struct attr_check` with two elements (because
+  we are checking two attributes):
+
+------------
+static struct attr_check *check;
+static void setup_check(void)
+{
+	if (check)
+		return; /* already done */
+	check = attr_check_initl("crlf", "ident", NULL);
+}
+------------
+
+. Call `git_check_attr()` with the prepared `struct attr_check`:
+
+------------
+	const char *path;
+
+	setup_check();
+	git_check_attr(path, check);
+------------
+
+. Act on `.value` member of the result, left in `check->items[]`:
+
+------------
+	const char *value = check->items[0].value;
+
+	if (ATTR_TRUE(value)) {
+		The attribute is Set, by listing only the name of the
+		attribute in the gitattributes file for the path.
+	} else if (ATTR_FALSE(value)) {
+		The attribute is Unset, by listing the name of the
+		attribute prefixed with a dash - for the path.
+	} else if (ATTR_UNSET(value)) {
+		The attribute is neither set nor unset for the path.
+	} else if (!strcmp(value, "input")) {
+		If none of ATTR_TRUE(), ATTR_FALSE(), or ATTR_UNSET() is
+		true, the value is a string set in the gitattributes
+		file for the path by saying "attr=value".
+	} else if (... other check using value as string ...) {
+		...
+	}
+------------
+
+To see how attributes in argv[] are set for different paths, only
+the first step in the above would be different.
+
+------------
+static struct attr_check *check;
+static void setup_check(const char **argv)
+{
+	check = attr_check_alloc();
+	while (*argv) {
+		struct git_attr *attr = git_attr(*argv);
+		attr_check_append(check, attr);
+		argv++;
+	}
+}
+------------
+
+
+Querying All Attributes
+-----------------------
+
+To get the values of all attributes associated with a file:
+
+* Prepare an empty `attr_check` structure by calling
+  `attr_check_alloc()`.
+
+* Call `git_all_attrs()`, which populates the `attr_check`
+  with the attributes attached to the path.
+
+* Iterate over the `attr_check.items[]` array to examine
+  the attribute names and values.  The name of the attribute
+  described by an `attr_check.items[]` object can be retrieved via
+  `git_attr_name(check->items[i].attr)`.  (Please note that no items
+  will be returned for unset attributes, so `ATTR_UNSET()` will return
+  false for all returned `attr_check.items[]` objects.)
+
+* Free the `attr_check` struct by calling `attr_check_free()`.