convert: return remote branches in git source
When converting git repos, all stuff happening on branches
seems to be ignored.
This is caused by the fact a "git clone" of a remote git
repo has all its branches prefixed with "origin/". By
chance, the "origin/master" branch is always linked to a
local "master" branch. So getheads() returns only the
master head, and it ignores all the other heads.
Make sure getheads() returns all heads, forcing remote
branches to be return by git-rev-parse.