When completing `git show origin/maint:Makef<tab>` we should add a
space after the filename has been completed, so that the user can
immediately begin the next argument.

I also added a special case for the symlink variant so we treat it
just like a normal blob, as there are no items below it in the Git
tree structure.
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>