gitk: Make "git describe" output clickable, too

gitk: Make "git describe" output clickable, too

Automake's contribution guidelines suggest using "git describe" output
in commit logs to reference previous commits.  By contrast, in
coreutils, I had acquired the habit of using a bare SHA1 prefix (8 hex
digits), since gitk creates clickable links for that, and not for "git
describe" output.

I prefer the readability of the full "git describe" output, yet want
to retain the gitk links, so this renders as clickable not just
SHA1-like strings, but also an SHA1-like string that is prefixed by
"-g".

Signed-off-by: Jim Meyering <meyering@redhat.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>