git-p4: Added support for automatically importing newly appearing perforce branches.

git-p4: Added support for automatically importing newly appearing perforce branches.

If a change in a p4 "branch" appears that hasn't seen any previous commit and
that has a known branch mapping we now try to import it properly. First we
find the p4 change of the source branch that the new p4 branch is based on. Then
we using git rev-list --bisect to locate the corresponding git commit to that change.
Finally we import all changes in the new p4 branch up to the current change and resume
with the regular import.

Signed-off-by: Simon Hausmann <simon@lst.de>