gitk: Use show-ref instead of ls-remote
gitk: Use show-ref instead of ls-remote

It used to be ls-remote on self was the only easy way to grab
the ref information.  Now we have show-ref which does not
involve fork and IPC, so use it.

Signed-off-by: Junio C Hamano <dc50d1021234060e53ec42a77d526afa2fe07479@cox.net>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git