backups: make hg backups evolve aware

Summary:
backups: make hg backups evolve aware
With this patch: hg backups urges evolve user to look at hg reflog to recover
hidden commits. This is because evolve's commands are using obsolescence
markers to hide commits instead of stripping them.

Test Plan: Cross repo dependency, not sure how to make an automated test

Reviewers: durham, pyd

Differential Revision: https://phabricator.fb.com/D2088486

Tasks: 6534705

backups.py

@@ -19,6 +19,11 @@ pager.attended.append('backups')
 cmdtable = {}
 command = cmdutil.command(cmdtable)
 testedwith = 'internal'
+msgwithevolve = """Evolve is enabled so no commit should be stripped unless you
+explicitely called hg strip. hg backups will show you the stripped commits.
+If you are trying to recover a commit hidden from a previous command, use hg 
+reflog to get its sha1 and you will be able to access it directly without 
+recovering a backup."""
 
 @command('^backups', [
     ('', 'recover', '', 'brings the specified commit back into the repository')
@@ -35,6 +40,13 @@ def backups(ui, repo, *pats, **opts):
     --verbose will print the entire commit message and the bundle path for that
     backup.
     '''
+    try:
+        if extensions.find('evolve'):
+            # Warn evolve users that they probably don't want to use backups
+            # but reflog instead
+            ui.warn(msgwithevolve)
+    except KeyError:
+        pass
     backuppath = repo.join("strip-backup")
     backups = filter(os.path.isfile, glob.glob(backuppath + "/*.hg"))
     backups.sort(key=lambda x: os.path.getmtime(x), reverse=True)