Summary:
Adds a message for users to use 'hg checkout --continue' if there's a
.hg/updatestate file (indicating an aborted checkout) and if they're on the null
rev (indicating they likely just cloned).
Also adds support for 'hg checkout --continue' to work with non-merge commits.
Note, it really only currently works when checking out from null, since
otherwise there will be a lot of modified files in the way. Once native checkout
is more mature, we can teach it to ignore modified files that match the desired
checkout destination.
Reviewed By: quark-zju
Differential Revision: D26967976
fbshipit-source-id: 7397c5fe82027e22bf1b4db0f14bb180239fae25
