revert: extract actual revert in its own function

With this small refactoring, the computation of the action to perform and the
actual change to the disk happen in different function. This allows extension to
wrap the later in case of need. The initial motivation is the `remotefilelog`
extension that need to prefetch all the file content to be checked out.

mercurial/cmdutil.py

@@ -2195,8 +2195,26 b' def revert(ui, repo, ctx, parents, *pats'
                         handle(revert, False)
                 else:
                     handle(remove, False)
+        if not opts.get('dry_run'):
+            _performrevert(repo, parents, ctx, revert, add, remove, undelete)
 
-        if not opts.get('dry_run'):
+            if targetsubs:
+                # Revert the subrepos on the revert list
+                for sub in targetsubs:
+                    ctx.sub(sub).revert(ui, ctx.substate[sub], *pats, **opts)
+    finally:
+        wlock.release()
+
+def _performrevert(repo, parents, ctx, revert, add, remove, undelete):
+    """function that actually perform all the action computed for revert
+
+    This is an independent function to let extension to plug in and react to
+    the imminent revert.
+
+    Make sure you have the working directory locked when caling this function.
+    """
+    parent, p2 = parents
+    node = ctx.node()
             def checkout(f):
                 fc = ctx[f]
                 repo.wwrite(f, fc.data(), fc.flags())
@@ -2244,13 +2262,6 b' def revert(ui, repo, ctx, parents, *pats'
                 if f in copied:
                     repo.dirstate.copy(copied[f], f)
 
-            if targetsubs:
-                # Revert the subrepos on the revert list
-                for sub in targetsubs:
-                    ctx.sub(sub).revert(ui, ctx.substate[sub], *pats, **opts)
-    finally:
-        wlock.release()
-
 def command(table):
     '''returns a function object bound to table which can be used as
     a decorator for populating table as a command table'''