git-gui: Avoid unnecessary symbolic-ref call during checkout
If we are checking out the branch we are already on then there is no need to call symbolic-ref to update the HEAD pointer to the "new" branch name, it is already correct.
Currently this situation does not happen very often, but it can be seen in some workflows where the user always recreates their local branch from a remote tracking branch and more-or-less ignores what branch he/she is on right now. As they say, ignorance is bliss.
This case will however become a tad more common when we overload checkout_op to actually also perform all of our merges. In that case we will likely see that the branch we want to "checkout" is the current branch, as we are actually just merging into it.
Signed-off-by: Shawn O. Pearce <firstname.lastname@example.org>