[PATCH v2 3/7] check-ref-format --branch: give Porcelain a way to grok branch shorthand

[Junio C Hamano <gitster@pobox.com>]

The command may not be the best place to add this new feature, but

    $ git check-ref-format --branch "@{-1}"

allows Porcelains to figure out what branch you were on before the last
branch switching.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 Documentation/git-check-ref-format.txt |   12 ++++++++++++
 builtin-check-ref-format.c             |   10 ++++++++++
 2 files changed, 22 insertions(+), 0 deletions(-)

diff --git a/Documentation/git-check-ref-format.txt b/Documentation/git-check-ref-format.txt
index 034223c..51579f6 100644
--- a/Documentation/git-check-ref-format.txt
+++ b/Documentation/git-check-ref-format.txt
@@ -7,7 +7,9 @@ git-check-ref-format - Make sure ref name is well formed
 
 SYNOPSIS
 --------
+[verse]
 'git check-ref-format' <refname>
+'git check-ref-format' [--branch] <branchname-shorthand>
 
 DESCRIPTION
 -----------
@@ -49,6 +51,16 @@ refname expressions (see linkgit:git-rev-parse[1]).  Namely:
   It may also be used to select a specific object such as with
   'git-cat-file': "git cat-file blob v1.3.3:refs.c".
 
+With the `--branch` option, it expands a branch name shorthand and
+prints the name of the branch the shorthand refers to.
+
+EXAMPLE
+-------
+
+git check-ref-format --branch @{-1}::
+
+Print the name of the previous branch.
+
 
 GIT
 ---
diff --git a/builtin-check-ref-format.c b/builtin-check-ref-format.c
index 701de43..39db6cb 100644
--- a/builtin-check-ref-format.c
+++ b/builtin-check-ref-format.c
@@ -5,9 +5,19 @@
 #include "cache.h"
 #include "refs.h"
 #include "builtin.h"
+#include "strbuf.h"
 
 int cmd_check_ref_format(int argc, const char **argv, const char *prefix)
 {
+	if (argc == 3 && !strcmp(argv[1], "--branch")) {
+		struct strbuf sb = STRBUF_INIT;
+		strbuf_branchname(&sb, argv[2]);
+		strbuf_splice(&sb, 0, 0, "refs/heads/", 11);
+		if (check_ref_format(sb.buf))
+			die("'%s' is not a valid branch name", argv[2]);
+		printf("%s\n", sb.buf + 11);
+		exit(0);
+	}
 	if (argc != 2)
 		usage("git check-ref-format refname");
 	return !!check_ref_format(argv[1]);
-- 
1.6.2.1.349.ga64c