filter: add GIT_FILTER_NO_SYSTEM_ATTRIBUTES option

Allow system-wide attributes (the ones specified in
`/etc/gitattributes`) to be ignored if the flag
`GIT_FILTER_NO_SYSTEM_ATTRIBUTES` is specified.

include/git2/filter.h

@@ -43,6 +43,9 @@ typedef enum {
 
 	/** Don't error for `safecrlf` violations, allow them to continue. */
 	GIT_FILTER_ALLOW_UNSAFE = (1u << 0),
+
+	/** Don't load `/etc/gitattributes` (or the system equivalent) */
+	GIT_FILTER_NO_SYSTEM_ATTRIBUTES = (1u << 1),
 } git_filter_flag_t;
 
 /**

src/filter.c

@@ -428,13 +428,18 @@ static int filter_list_check_attributes(
 	git_filter_def *fdef,
 	const git_filter_source *src)
 {
-	int error;
-	size_t i;
 	const char **strs = git__calloc(fdef->nattrs, sizeof(const char *));
+	uint32_t flags = 0;
+	size_t i;
+	int error;
+
 	GIT_ERROR_CHECK_ALLOC(strs);
 
+	if ((src->flags & GIT_FILTER_NO_SYSTEM_ATTRIBUTES) != 0)
+		flags |= GIT_ATTR_CHECK_NO_SYSTEM;
+
 	error = git_attr_get_many_with_session(
-		strs, repo, attr_session, 0, src->path, fdef->nattrs, fdef->attrs);
+		strs, repo, attr_session, flags, src->path, fdef->nattrs, fdef->attrs);
 
 	/* if no values were found but no matches are needed, it's okay! */
 	if (error == GIT_ENOTFOUND && !fdef->nmatches) {