symbolic-ref --short: abbreviate the output unambiguously

It can be helpful to resolve a symbolic ref and output the result in a
shortened form, such as for use in shell prompts.  Add a "--short" option
to do so.

Signed-off-by: Jan Krüger <jk@jk.gs>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-symbolic-ref.txt

@@ -8,7 +8,8 @@ git-symbolic-ref - Read and modify symbolic refs
 SYNOPSIS
 --------
 [verse]
-'git symbolic-ref' [-q] [-m <reason>] <name> [<ref>]
+'git symbolic-ref' [-m <reason>] <name> <ref>
+'git symbolic-ref' [-q] [--short] <name>
 
 DESCRIPTION
 -----------
@@ -33,6 +34,10 @@ OPTIONS
 	symbolic ref but a detached HEAD; instead exit with
 	non-zero status silently.
 
+--short::
+	When showing the value of <name> as a symbolic ref, try to shorten the
+	value, e.g. from `refs/heads/master` to `master`.
+
 -m::
 	Update the reflog for <name> with <reason>.  This is valid only
 	when creating or updating a symbolic ref.

builtin/symbolic-ref.c

@@ -8,21 +8,25 @@ static const char * const git_symbolic_ref_usage[] = {
 	NULL
 };
 
+static int shorten;
+
 static void check_symref(const char *HEAD, int quiet)
 {
 	unsigned char sha1[20];
 	int flag;
-	const char *refs_heads_master = resolve_ref_unsafe(HEAD, sha1, 0, &flag);
+	const char *refname = resolve_ref_unsafe(HEAD, sha1, 0, &flag);
 
-	if (!refs_heads_master)
+	if (!refname)
 		die("No such ref: %s", HEAD);
 	else if (!(flag & REF_ISSYMREF)) {
 		if (!quiet)
 			die("ref %s is not a symbolic ref", HEAD);
 		else
 			exit(1);
 	}
-	puts(refs_heads_master);
+	if (shorten)
+		refname = shorten_unambiguous_ref(refname, 0);
+	puts(refname);
 }
 
 int cmd_symbolic_ref(int argc, const char **argv, const char *prefix)
@@ -32,6 +36,7 @@ int cmd_symbolic_ref(int argc, const char **argv, const char *prefix)
 	struct option options[] = {
 		OPT__QUIET(&quiet,
 			"suppress error message for non-symbolic (detached) refs"),
+		OPT_BOOL(0, "short", &shorten, "shorten ref output"),
 		OPT_STRING('m', NULL, &msg, "reason", "reason of the update"),
 		OPT_END(),
 	};