fix issue where REVSPEC~N is not equivalent to skipping N commits at the beginning
return total count of commits in Commits
Make malformed hg committer names/emails non-fatal
Add Diff method on Repository, with git and hg cmd impls
cross-repo diffs
diff for libgit2 impl
basic cmd for testing/dev
add "show" subcmd
topo sort in libgit2 commits list (matches `git log`)
return Err{Revision,Branch,Tag}NotFound errors
Use vfs.FileSystem instead of own interface.

Closes #11.
Merge pull request #12 from shurcooL/master

Use vfs.FileSystem instead of own interface.
lenient hg date parsing
Merge pull request #15 from sourcegraph/lenient-hg-date-parsing

lenient hg date parsing
use cmdline hg impl for filesystem access to work around bugs
ssh git WIP