commit: print debug message when clearing dirstate and wdir clean

This case is a little weird, so let's have a debug message.

Differential Revision: https://phab.mercurial-scm.org/D8231

mercurial/localrepo.py

@@ -2956,6 +2956,7 b' class localrepository(object):'
                 or self.ui.configbool(b'ui', b'allowemptycommit')
             )
             if not allowemptycommit:
+                self.ui.debug(b'nothing to commit, clearing merge state\n')
                 ms.reset()
                 return None
 

tests/test-graft.t

@@ -758,6 +758,7 b' Transplants of grafts can find a destina'
    branchmerge: True, force: True, partial: False
    ancestor: b592ea63bb0c, local: 7e61b508e709+, remote: 7a4785234d87
   starting 4 threads for background file closing (?)
+  nothing to commit, clearing merge state
   note: graft of 13:7a4785234d87 created no changes to commit
   $ hg log -r 'destination(13)'
 All copies of a cset