git-tracker: restrict shortlog to existing files

git-tracker: restrict shortlog to existing files

When the git-tracker updates through a merge commit or
otherwise multiple kernel commits, it appends a shortlog
of all the changes. That's fine, but is often unhelpful
since it's so large - restrict it to the files that are
actually backported to make it more readable.

While at it, rewrite the bpgit.status() helper function
to actually do something useful.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>