Improve Arcanist + Windows + SVN compatibility

Summary: From "cmd.exe" with, e.g. SilkSVN, there are some issues getting arc to do anything useful. Resolve enough of them so that it's at least usable.

Test Plan: Created a revision from Windows / cmd.exe / arc / SVN.

Reviewers: btrahan, jungejason, vrana

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D2984