Fix 'arc lint --everything' when svn has uncommitted changes

Summary: My recent change adding --everything to arc lint could sometimes cause a "diff is empty" error, this patch fixes it.

Test Plan: Ran "arc lint --everything" before and after patch. No longer errors out. Only appeared to originally happen when there were uncommited changes in an svn repo.

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, aran

Differential Revision: https://secure.phabricator.com/D6732
fixes local svn branch name detection

Summary: Corrects relative vs absolute branch name when using 'arc commit'

Test Plan: 'arc commit' on a release branch gave an error before making the change (change was generated from 'branches/yyy' but working copy root is 'https://xxx/branches/yyy', now it does not.

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, aran

Differential Revision: https://secure.phabricator.com/D6743
Fix arc land for mercurial on windows

Summary:
arc for mercurial on windows was broken in several way.
Executing a command via passthru failed because passthru on windows
skips the shell so 'set HGPLAIN=1 & ...' was an invalid command. The
fix was to just not set HGPLAIN for passthru commands on windows.

Also removed hardcoded '' quotes in mercurial commands since windows
doesn't support single quots.

Test Plan: arc land --hold on a windows machine

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, aran

Differential Revision: https://secure.phabricator.com/D6763
Merge branch 'master' of http://github.com/facebook/arcanist