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>

pathspec.h

@@ -3,7 +3,10 @@
 
 /* Pathspec magic */
 #define PATHSPEC_FROMTOP	(1<<0)
-#define PATHSPEC_ALL_MAGIC PATHSPEC_FROMTOP
+#define PATHSPEC_MAXDEPTH	(1<<1)
+#define PATHSPEC_ALL_MAGIC	  \
+	(PATHSPEC_FROMTOP	| \
+	 PATHSPEC_MAXDEPTH)
 
 #define PATHSPEC_ONESTAR 1	/* the pathspec pattern sastisfies GFNM_ONESTAR */
 
@@ -27,6 +30,7 @@ struct pathspec {
 /* parse_pathspec flags */
 #define PATHSPEC_PREFER_CWD (1<<0) /* No args means match cwd */
 #define PATHSPEC_PREFER_FULL (1<<1) /* No args means match everything */
+#define PATHSPEC_MAXDEPTH_VALID (1<<2) /* max_depth field is valid */
 
 extern int init_pathspec(struct pathspec *, const char **);
 extern void parse_pathspec(struct pathspec *pathspec,