git-archimport: allow remapping branch names

git-archimport: allow remapping branch names

This patch adds support to archimport for remapping the branch
names to match those used in git more closely.  This is useful
for projects that migrate to git (as opposed to users that want
to use git on Arch-based projects).  For example, one can choose
an Arch branch name and call it "master".

The new command-line syntax works even if there is a colon in
a branch name, since only the part after the last colon is taken
to be the git name (git does not allow colons in branch names).

The new feature is implemented so that archives rotated every
year can also be remapped into a single git archive.

Signed-off-by: Paolo Bonzini  <bonzini@gnu.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>