commit: factor out empty commit check to `basectx.isempty()`

This enables reuse in other places, e.g. those dealing with `memctx`.

Differential Revision: https://phab.mercurial-scm.org/D8729

mercurial/context.py

@@ -481,6 +481,14 b' class basectx(object):'
             '%s does not implement mergestate()' % self.__class__
         )
 
+    def isempty(self):
+        return not (
+            len(self.parents()) > 1
+            or self.branch() != self.p1().branch()
+            or self.closesbranch()
+            or self.files()
+        )
+
 
 class changectx(basectx):
     """A changecontext object makes access to data related to a particular

mercurial/localrepo.py

@@ -2995,14 +2995,9 b' class localrepository(object):'
             mergeutil.checkunresolved(ms)
 
             # internal config: ui.allowemptycommit
-            allowemptycommit = (
-                cctx.branch() != cctx.p1().branch()
-                or extra.get(b'close')
-                or merge
-                or cctx.files()
-                or self.ui.configbool(b'ui', b'allowemptycommit')
-            )
-            if not allowemptycommit:
+            if cctx.isempty() and not self.ui.configbool(
+                b'ui', b'allowemptycommit'
+            ):
                 self.ui.debug(b'nothing to commit, clearing merge state\n')
                 ms.reset()
                 return None