abump: add flag -b to create git branch before bumping

Flag to create branches named "bump-name-ver". If several packages are requested, each is bumped in a separate branch, all starting from the currently checked out branch. Implements #10166 (closed)

Examples creating branch "bump-pkgname-1.2.3" and bump pkgname:

# short
abump -b pkgname-1.2.3
# long
abump --branch pkgname-1.2.3
# explicit separation of positional arguments
abump -b -- pkgname-1.2.3

Caveat, bumping two packages creates one branch "bump-pkgname-1.2.3":

abump -b pkgname-1.2.3 namedpkg-3.2.1

Users must still remember to first switch to and pull from the destination branch, i.e. master or a release branch, for security fixes.