parse_pathspec: add special flag for max_depth feature

match_pathspec_depth() and tree_entry_interesting() check max_depth
field in order to support "git grep --max-depth". The feature
activation is tied to "recursive" field, which led to some unwanted
activation, e.g. 5c8eeb8 (diff-index: enable recursive pathspec
matching in unpack_trees - 2012-01-15).

This patch decouples the activation from "recursive" field, puts it in
"magic" field instead. This makes sure that only "git grep" can
activate this feature. And because parse_pathspec knows when the
feature is not used, it does not need to sort pathspec (required for
max_depth to work correctly). A small win for non-grep cases.

Even though a new magic flag is introduced, no magic syntax is. The
magic can be only enabled by parse_pathspec() caller. We might someday
want to support ":(maxdepth:10)src." It all depends on actual use
cases.

max_depth feature cannot be enabled via init_pathspec() anymore. But
that's ok because init_pathspec() is on its way to /dev/null.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/grep.c

@@ -858,7 +858,8 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 	}
 
 	parse_pathspec(&pathspec, 0,
-		       PATHSPEC_PREFER_CWD,
+		       PATHSPEC_PREFER_CWD |
+		       (opt.max_depth != -1 ? PATHSPEC_MAXDEPTH_VALID : 0),
 		       prefix, argv + i);
 	pathspec.max_depth = opt.max_depth;
 	pathspec.recursive = 1;