git-gui: Don't crash in citool mode on initial commit.

git-gui: Don't crash in citool mode on initial commit.

Attempting to use `git citool` to create an initial commit caused
git-gui to crash with a Tcl error as it tried to add the newly
born branch to the non-existant branch menu.  Moving this code
to after the normal commit cleanup logic resolves the issue, as
we only have a branch menu if we are not in singlecommit mode.

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