git-gui: Don't delete the test target branch.

git-gui: Don't delete the test target branch.

Its possible for the user to select a branch for the merge test
(while deleting branches) and also select that branch for deletion.
Doing so would have bypassed our merge check for that branch, as
a branch is always a strict subset of itself.  So we will simply
skip over a branch and not delete it if that is the branch which
the user selected for the merge check.

Signed-off-by: Shawn O. Pearce <redacted>