Previously, the binaries were only updated if the version number had
changed. Now they're updated if just the commit changes. This also
improves the output of "pub upgrade" if a git dependency changes its
commit but not its version number.

R=rnystrom@google.com
BUG=21971

Review URL: https://codereview.chromium.org//844263004

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42883 260f80e4-7a28-3924-810f-c04153c831b5