pull: suppress error when no remoteref is found

Commit 48059e4 (pull: use merge-base --fork-point when appropriate,
2013-12-08) incorrectly assumes that get_remote_merge_branch will either
yield a non-empty string or return an error, but there are circumstances
where it will yield an empty string.

The previous code then invoked git-rev-list with no arguments, which
results in an error suppressed by redirecting stderr to /dev/null.  Now
we invoke git-merge-base with an empty branch name, which also results
in an error.  Suppress this in the same way.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>