git: support '{files}' template

Summary:
Change `ctx.files()` to run diff for git repos. This would make a few revsets
and templates working.

Reviewed By: danielocfb

Differential Revision: D33354722

fbshipit-source-id: c6449d7f405f2d2c568294eeebe47d371e46cb12

eden/scm/edenscm/mercurial/context.py

@@ -25,6 +25,7 @@ from . import (
     error,
     extensions,
     fileset,
+    git,
     match as matchmod,
     mdiff,
     mutation,
@@ -576,7 +577,11 @@ class changectx(basectx):
         return self._changeset.date
 
     def files(self):
-        return self._changeset.files
+        files = self._changeset.files
+        # git does not provide "files" in commit message - run diff to get it
+        if not files and git.isgit(self._repo):
+            files = sorted(self.manifest().diff(self.p1().manifest()).keys())
+        return files
 
     def description(self):
         return self._changeset.description

eden/scm/tests/test-git.t

@@ -289,3 +289,8 @@ Test push:
   abort: use '--to' to specify destination bookmark
   [255]
 
+"files" metadata:
+
+  $ hg log -r $A+$B -T '{files}\n'
+  A
+  B