rebase: correctly check for empty commit in in-memory mode

The new code has a small overhead in the empty commit case, as a `memctx` object
is always created, but I don’t think it’s justified here to duplicate code
to optimize a relatively unlikely code path.

Differential Revision: https://phab.mercurial-scm.org/D8732

hgext/rebase.py

@@ -1427,12 +1427,6 b' def externalparent(repo, state, destance'
 def commitmemorynode(repo, wctx, editor, extra, user, date, commitmsg):
     '''Commit the memory changes with parents p1 and p2.
     Return node of committed revision.'''
-    # FIXME: make empty commit check consistent with ``repo.commit``
-    if wctx.nofilechanges() and not repo.ui.configbool(
-        b'ui', b'allowemptycommit'
-    ):
-        return None
-
     # By convention, ``extra['branch']`` (set by extrafn) clobbers
     # ``branch`` (used when passing ``--keepbranches``).
     branch = None
@@ -1447,6 +1441,8 b' def commitmemorynode(repo, wctx, editor,'
         branch=branch,
         editor=editor,
     )
+    if memctx.isempty() and not repo.ui.configbool(b'ui', b'allowemptycommit'):
+        return None
     commitres = repo.commitctx(memctx)
     wctx.clean()  # Might be reused
     return commitres

tests/test-rebase-inmemory.t

@@ -949,14 +949,11 b' definition.'
   |/
   o  0: d20a80d4def3 'base'
   
-FIXME: It's broken for inmemory merges.
   $ hg rebase -s 2 -d 3
   rebasing 2:0194f1db184a "b"
   note: not rebasing 2:0194f1db184a "b", its destination already has all its changes
   rebasing 4:59c8292117b1 "merge" (tip)
-  note: not rebasing 4:59c8292117b1 "merge" (tip), its destination already has all its changes (true !)
   saved backup bundle to $TESTTMP/keep_merge/.hg/strip-backup/0194f1db184a-aee31d03-rebase.hg
-#if false
   $ hg tglog
   o    3: 506e2454484b 'merge'
   |\
@@ -966,4 +963,3 b" FIXME: It's broken for inmemory merges."
   |/
   o  0: d20a80d4def3 'base'
   
-#endif