subrepo: only retrieve the first two components of the Git version

This makes the version detection compatible with Git versions like '1.9-rc0'.
We only cared about the first two components of the version anyway.

--HG--
branch : stable
extra : rebase_source : 8d362f4c81781f3b8d51f41e74775f83a697965b
subrepo: convert matched string to integer before comparing Git version

(1, '4') is greater than (1, 5) so the version check never actually worked.

--HG--
branch : stable
extra : rebase_source : 8b5081a4f515e6295aa51aa63e7eaeaad2b42da7
subrepo: add trailing newlines to warnings

--HG--
branch : stable
extra : rebase_source : 38e1420392ad092d73dab6dc90dba6b6282c5be6