rev-parse: --abbrev-ref option to shorten ref name

This applies the shorten_unambiguous_ref function to the object name.
Default mode is controlled by core.warnAmbiguousRefs. Else it is given as
optional argument to --abbrev-ref={strict|loose}.

This should be faster than 'git for-each-ref --format="%(refname:short)" <ref>'
for single refs.

Signed-off-by: Bert Wesarg <bert.wesarg@googlemail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-rev-parse.txt

@@ -84,6 +84,11 @@ OPTIONS
 	unfortunately named tag "master"), and show them as full
 	refnames (e.g. "refs/heads/master").
 
+--abbrev-ref[={strict|loose}]::
+	A non-ambiguous short name of the objects name.
+	The option core.warnAmbiguousRefs is used to select the strict
+	abbreviation mode.
+
 --all::
 	Show all refs found in `$GIT_DIR/refs`.
 

builtin-rev-parse.c

@@ -26,6 +26,8 @@ static int show_type = NORMAL;
 #define SHOW_SYMBOLIC_FULL 2
 static int symbolic;
 static int abbrev;
+static int abbrev_ref;
+static int abbrev_ref_strict;
 static int output_sq;
 
 /*
@@ -109,8 +111,8 @@ static void show_rev(int type, const unsigned char *sha1, const char *name)
 		return;
 	def = NULL;
 
-	if (symbolic && name) {
-		if (symbolic == SHOW_SYMBOLIC_FULL) {
+	if ((symbolic || abbrev_ref) && name) {
+		if (symbolic == SHOW_SYMBOLIC_FULL || abbrev_ref) {
 			unsigned char discard[20];
 			char *full;
 
@@ -125,6 +127,9 @@ static void show_rev(int type, const unsigned char *sha1, const char *name)
 				 */
 				break;
 			case 1: /* happy */
+				if (abbrev_ref)
+					full = shorten_unambiguous_ref(full,
+						abbrev_ref_strict);
 				show_with_type(type, full);
 				break;
 			default: /* ambiguous */
@@ -506,6 +511,20 @@ int cmd_rev_parse(int argc, const char **argv, const char *prefix)
 				symbolic = SHOW_SYMBOLIC_FULL;
 				continue;
 			}
+			if (!prefixcmp(arg, "--abbrev-ref") &&
+			    (!arg[12] || arg[12] == '=')) {
+				abbrev_ref = 1;
+				abbrev_ref_strict = warn_ambiguous_refs;
+				if (arg[12] == '=') {
+					if (!strcmp(arg + 13, "strict"))
+						abbrev_ref_strict = 1;
+					else if (!strcmp(arg + 13, "loose"))
+						abbrev_ref_strict = 0;
+					else
+						die("unknown mode for %s", arg);
+				}
+				continue;
+			}
 			if (!strcmp(arg, "--all")) {
 				for_each_ref(show_reference, NULL);
 				continue;