Add a test for the case when only one parameter is passed to '-m'
(move/rename) option.

For example - if 'git branch -m bbb' is run while checked out on aaa
branch, it should rename the currently checked out branch to bbb.
There was no test for this particular case with only one parameter
for -m option. However, there's one similar test case for -M option.

Add test for making sure HEAD points to the bbb (new branch name). Also
add a test for making sure the reflog that is moved to 'bbb' retains
entries created for the currently checked out branch. Note that since
the topmost entry on reflog for bbb will be about branch creation, we
compare bbb@{1} (instead of bbb@{0}) with aaa@{0} to make sure the
reflog for bbb retains entries from aaa.
Signed-off-by: NSahil Dua <sahildua2305@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>