Restrict the tags used to generate the version string to those that
begin with "v", since git's tags for git-core (ie. excluding git-gui)
are all of the form "vX.Y...".
This is to avoid using private tags by the user in a clone of the git
code repository, which may break certain machinery (eg. Makefile, gitk).
Signed-off-by: Tay Ray Chuan <rctay89@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>