Teach "git describe" --dirty option

With the --dirty option, git describe works on HEAD but append s"-dirty"
iff the contents of the work tree differs from HEAD.  E.g.

  $ git describe --dirty
  v1.6.5-15-gc274db7
  $ echo >> Makefile
  $ git describe --dirty
  v1.6.5-15-gc274db7-dirty

The --dirty option can also be used to specify what is appended, instead
of the default string "-dirty".

  $ git describe --dirty=.mod
  v1.6.5-15-gc274db7.mod

Many build scripts use `git describe` to produce a version number based on
the description of HEAD (on which the work tree is based) + saying that if
the build contains uncommitted changes.  This patch helps the writing of
such scripts since `git describe --dirty` does directly the intended thing.

Three possiblities were considered while discussing this new feature:

1. Describe the work tree by default and describe HEAD only if "HEAD" is
   explicitly specified

     Pro: does the right thing by default (both for users and for scripts)
     Pro: other git commands that works on the work tree by default
     Con: breaks existing scripts used by the Linux kernel and other projects

2. Use --worktree instead of --dirty

     Pro: does what it says: "git describe --worktree" describes the work tree
     Con: other commands do not require a --worktree option when working
          on the work tree (it often is the default mode for them)
     Con: unusable with an optional value: "git describe --worktree=.mod"
	  is quite unintuitive.

3. Use --dirty as in this patch

     Pro: makes sense to specify an optional value (what the dirty mark is)
     Pro: does not have any of the big cons of previous alternatives
	  * does not break scripts
	  * is not inconsistent with other git commands

This patch takes the third approach.

Signed-off-by: Jean Privat <jean@pryen.org>
Acked-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-describe.c

@@ -5,12 +5,14 @@
 #include "builtin.h"
 #include "exec_cmd.h"
 #include "parse-options.h"
+#include "diff.h"
 
 #define SEEN		(1u<<0)
 #define MAX_TAGS	(FLAG_BITS - 1)
 
 static const char * const describe_usage[] = {
 	"git describe [options] <committish>*",
+	"git describe [options] --dirty",
 	NULL
 };
 
@@ -23,6 +25,13 @@ static int max_candidates = 10;
 static int found_names;
 static const char *pattern;
 static int always;
+static const char *dirty;
+
+/* diff-index command arguments to check if working tree is dirty. */
+static const char *diff_index_args[] = {
+	"diff-index", "--quiet", "HEAD", "--", NULL
+};
+
 
 struct commit_name {
 	struct tag *tag;
@@ -208,6 +217,8 @@ static void describe(const char *arg, int last_one)
 		display_name(n);
 		if (longformat)
 			show_suffix(0, n->tag ? n->tag->tagged->sha1 : sha1);
+		if (dirty)
+			printf("%s", dirty);
 		printf("\n");
 		return;
 	}
@@ -265,7 +276,10 @@ static void describe(const char *arg, int last_one)
 	if (!match_cnt) {
 		const unsigned char *sha1 = cmit->object.sha1;
 		if (always) {
-			printf("%s\n", find_unique_abbrev(sha1, abbrev));
+			printf("%s", find_unique_abbrev(sha1, abbrev));
+			if (dirty)
+				printf("%s", dirty);
+			printf("\n");
 			return;
 		}
 		die("cannot describe '%s'", sha1_to_hex(sha1));
@@ -300,6 +314,8 @@ static void describe(const char *arg, int last_one)
 	display_name(all_matches[0].name);
 	if (abbrev)
 		show_suffix(all_matches[0].depth, cmit->object.sha1);
+	if (dirty)
+		printf("%s", dirty);
 	printf("\n");
 
 	if (!last_one)
@@ -324,6 +340,9 @@ int cmd_describe(int argc, const char **argv, const char *prefix)
 			   "only consider tags matching <pattern>"),
 		OPT_BOOLEAN(0, "always",     &always,
 			   "show abbreviated commit object as fallback"),
+		{OPTION_STRING, 0, "dirty",  &dirty, "mark",
+			   "append <mark> on dirty working tree (default: \"-dirty\")",
+		 PARSE_OPT_OPTARG, NULL, (intptr_t) "-dirty"},
 		OPT_END(),
 	};
 
@@ -360,7 +379,11 @@ int cmd_describe(int argc, const char **argv, const char *prefix)
 	}
 
 	if (argc == 0) {
+		if (dirty && !cmd_diff_index(ARRAY_SIZE(diff_index_args) - 1, diff_index_args, prefix))
+			dirty = NULL;
 		describe("HEAD", 1);
+	} else if (dirty) {
+		die("--dirty is incompatible with committishes");
 	} else {
 		while (argc-- > 0) {
 			describe(*argv++, argc == 0);