Allow fetching references from any namespace

Allow fetching references from any namespace

not only from the three defined: heads, tags and remotes.

Noticed when I tried to fetch the references created by git-p4-import.bat:
they are placed into separate namespace (refs/p4import/, to avoid showing
them in git-branch output). As canon_refs_list_for_fetch always prepended
refs/heads/ it was impossible, and annoying: it worked before. Normally,
the p4import references are useless anywhere but in the directory managed
by perforce, but in this special case the cloned directory was supposed
to be a backup, including the p4import branch: it keeps information about
where the imported perforce state came from.

Signed-off-by: Alex Riesen <raa.lkml@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>