overlayworkingctx: rename misleadingly named `isempty()` method

This method is only about whether there are file changes, not about whether the
commit will be empty or not.

One user of the method was incorrectly assuming the latter meaning, leading to
the bug for which a test case was added in D8727. I’ve added a FIXME to the
code.

The original motivation for the rename was that I want to add
`committablectx.isempty()`, that properly checks if a commit will be empty,
using the exact same logic as in `repo.commit()`, and I wanted to avoid a name
clash.

Differential Revision: https://phab.mercurial-scm.org/D8728

hgext/rebase.py

@@ -1427,8 +1427,10 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.'''
-    # Replicates the empty check in ``repo.commit``.
-    if wctx.isempty() and not repo.ui.configbool(b'ui', b'allowemptycommit'):
+    # 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

mercurial/context.py

@@ -2519,7 +2519,7 b' class overlayworkingctx(committablectx):'
     def isdirty(self, path):
         return path in self._cache
 
-    def isempty(self):
+    def nofilechanges(self):
         # We need to discard any keys that are actually clean before the empty
         # commit check.
         self._compact()