Use divided stdout and stderr output in ResolveRevision.
Use divided stdout and stderr output in ResolveRevision.

Use stdout for parsing command output. This eliminates possible
warnings from ruining output.
Use stderr for parsing error type and printing error message.
I'm not able to reproduce "unknown revision" text while running `git
rev-parse <bad-commit>` with my local git version; however I do get
"fatal: bad revision <bad-commit>" on *stderr* when I try `git
rev-parse <bad-commit> --`. That text does not appear in stdout.

alexsaveliev/go-vcs,pombredanne/go-vcs,pombredanne/go-vcs,alexsaveliev/go-vcs