tag: support --sort=<spec>

--sort=version:refname (or --sort=v:refname for short) sorts tags as
if they are versions. --sort=-refname reverses the order (with or
without ":version").

versioncmp() is copied from string/strverscmp.c in glibc commit
ee9247c38a8def24a59eb5cfb7196a98bef8cfdc, reformatted to Git coding
style. The implementation is under LGPL-2.1 and according to [1] I can
relicense it to GPLv2.

[1] http://www.gnu.org/licenses/gpl-faq.html#AllCompatibility

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/cache.h b/cache.h
index dc040fb..082a783 100644
--- a/cache.h
+++ b/cache.h
@@ -1367,4 +1367,6 @@ int stat_validity_check(struct stat_validity *sv, const char *path);
  */
 void stat_validity_update(struct stat_validity *sv, int fd);
 
+int versioncmp(const char *s1, const char *s2);
+
 #endif /* CACHE_H */