Use remote branch name for local branch if possible.

When setting up a local remote-tracking branch (either on clone or when
the user chooses a different branch in the rc file), kdesrc-build has
preferred a '$remoteName-$branchName' naming convention to try to avoid
stepping on user's existing branches.

In reality most users do not use local branches, and those that do would
(according to feedback I've received) prefer the local and remote branch
names to be the same as per git practice basically everywhere.

Since it's easy enough to check that the local branch doesn't already
exist before creating it, just do that and drop the remote name from the
created branch name.

BUG:294347
FIXED-IN:1.15