Allow git merge ":/<pattern>"

Allow git merge ":/<pattern>"

It probably is not such a good idea to use ":/<pattern>" to specify which
commit to merge, as ":/<pattern>" can often hit unexpected commits, but
somebody tried it and got a nonsense error message:

	fatal: ':/Foo bar' does not point to a commit

So here is a for-the-sake-of-consistency update that is fairly useless
that allows users to carefully try not shooting in the foot.

Signed-off-by: Junio C Hamano <gitster@pobox.com>