uncommit: allow power users to leave an empty commit around

Some power user workflow requires `uncommit` to leave an empty commit so
they can run `hg amend` afterwards. I don't find easy alternatives so let's
allow that if `ui.allowemptyconfig` is set.

Note: `ui.allowemptyconfig` is an internal config that is not documented for
end-users. Core Mercurial seems to be trying to prevent the empty commit
case from normal users (like, comment disallows that, rebase will skip a
commit if after rebasing it becomes empty). So it seems better to still use
that undocumented config instead of a more discoverable flag.

Differential Revision: https://phab.mercurial-scm.org/D156

hgext3rd/uncommit.py

@@ -57,7 +57,7 @@ def _commitfiltered(repo, ctx, match):
         return None
 
     files = (initialfiles - exclude)
-    if not files:
+    if not files and not repo.ui.configbool('ui', 'allowemptycommit'):
         return ctx.parents()[0].node()
 
     # Filter copies

tests/test-uncommit.t

@@ -5,6 +5,7 @@ Test uncommit - set up the config
   > evolution=createmarkers
   > [extensions]
   > uncommit = $TESTDIR/../hgext3rd/uncommit.py
+  > drawdag=$RUNTESTDIR/drawdag.py
   > EOF
 
 Build up a repo
@@ -200,3 +201,25 @@ Partial uncommit with public parent
   14: draft
   $ hg phase -r ".^"
   10: public
+
+Uncommit leaving an empty changeset
+
+  $ cd $TESTTMP
+  $ hg init repo1
+  $ cd repo1
+  $ hg debugdrawdag <<'EOS'
+  > Q
+  > |
+  > P
+  > EOS
+  $ hg up Q -q
+  $ hg uncommit --config ui.allowemptycommit=1
+  $ hg log -G -T '{desc} FILES: {files}'
+  @  Q FILES:
+  |
+  | x  Q FILES: Q
+  |/
+  o  P FILES: P
+  
+  $ hg status
+  A Q