convert/git: catch errors from modern git-ls-remote (issue3428)
Since git v1.7.8.2-327-g926f1dd (the change was first released in git
1.7.10), git does not return non-zero when "git ls-remote --tags ..."
is run and the repository is damaged. This causes the "damaged
repository with missing commit" test in test-convert-git.t to
unexpectedly succeed.
Fix by aborting if git outputs any lines beginning with "error:",
which required adding some subprocess use in convert/git.py.