Specify date format in git blame

Summary:
right now 'arc blame' uses the user's blame.date setting, but
it will only work when the format is iso. So it will fail for user who
has customized it.

Test Plan:
run arc cover with 'blame.date' set to relative and verified
that 'arc cover' still works

Reviewed By: epriestley
Reviewers: epriestley, codeblock
CC: hwang, aran, epriestley
Differential Revision: 745

src/repository/api/git/ArcanistGitAPI.php

@@ -317,7 +317,7 @@ class ArcanistGitAPI extends ArcanistRepositoryAPI {
   public function getBlame($path) {
     // TODO: 'git blame' supports --porcelain and we should probably use it.
     list($stdout) = execx(
-      '(cd %s; git blame -w -C %s -- %s)',
+      '(cd %s; git blame --date=iso -w -C %s -- %s)',
       $this->getPath(),
       $this->getRelativeCommit(),
       $path);