Fix NPE in MergeOp when submit to new branch fails due to dependencies

This resolves the stack trace reported in GERRIT-192, where we hit an
NPE because mergeTip was still null, as no changes were chosen to be
merged into the stillborn branch, due to missing dependencies.

Bug: GERRIT-192
Signed-off-by: Shawn O. Pearce <sop@google.com>
1 file changed