git-remote: fix "Use of uninitialized value in string ne"

git-remote: fix "Use of uninitialized value in string ne"

martin f krafft <madduck@madduck.net> writes:

> piper:~> git remote show origin
> * remote origin
>   URL: ssh://git.madduck.net/~/git/etc/mailplate.git
> Use of uninitialized value in string ne at /usr/local/stow/git/bin/git-remote line 248.

This is because there might not be branch.<name>.remote defined but
the code unconditionally dereferences $branch->{$name}{'REMOTE'} and
compares with another string.

Tested-by: Martin F Krafft <madduck@madduck.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>