subrepo: add revert support without backup for git subrepos

Previously, git subrepos did not support reverting.
This change adds basic support for reverting
when '--no-backup' is specified.
A warning is given (and the current state is kept)
when a revert is done without the '--no-backup' flag.

mercurial/subrepo.py

@@ -1656,6 +1656,17 b' class gitsubrepo(abstractsubrepo):'
         elif match(gitprefix): #Subrepo is matched
             ui.write(self._gitcommand(cmd))
 
+    def revert(self, ui, substate, *pats, **opts):
+        ui.status(_('reverting subrepo %s\n') % substate[0])
+        if not opts.get('no_backup'):
+            ui.warn('%s: reverting %s subrepos without '
+                    '--no-backup is unsupported\n'
+                    % (substate[0], substate[2]))
+            return []
+
+        self.get(substate, overwrite=True)
+        return []
+
     def shortid(self, revid):
         return revid[:7]
 

tests/test-subrepo-git.t

@@ -784,4 +784,21 b' ensure adding include/exclude ignores th'
   $ hg diff --subrepos -I s/foobar
   $ hg diff --subrepos -X s/foobar
 
+revert the subrepository
+  $ hg revert --all
+  reverting subrepo ../gitroot (glob)
+  ../gitroot: reverting git subrepos without --no-backup is unsupported (glob)
+
+  $ hg status --subrepos
+  M s/foobar
+  A s/barfoo
+
+  $ hg revert --no-backup --all
+  reverting subrepo ../gitroot (glob)
+  $ hg revert --no-backup s
+  reverting subrepo ../gitroot (glob)
+
+  $ hg status --subrepos
+  ? s/barfoo
+
   $ cd ..