amend: fix incompatibity between logfile and message option (issue3675)

Bug introduced by 9732473aa24b

mercurial/cmdutil.py

@@ -1611,6 +1611,10 b' def amend(ui, repo, commitfunc, old, ext'
             # See if we got a message from -m or -l, if not, open the editor
             # with the message of the changeset to amend
             message = logmessage(ui, opts)
+            # ensure logfile does not conflict with later enforcement of the
+            # message. potential logfile content has been processed by
+            # `logmessage` anyway.
+            opts.pop('logfile')
             # First, do a regular commit to record all changes in the working
             # directory (if there are any)
             ui.callhooks = False

tests/test-commit-amend.t

@@ -65,9 +65,11 b' Add new file:'
   saved backup bundle to $TESTTMP/.hg/strip-backup/43f1ba15f28a-amend-backup.hg (glob)
 
 Remove file that was added in amended commit:
+(and test logfile option)
 
   $ hg rm b
-  $ hg ci --amend -m 'amend base1 remove new file'
+  $ echo 'amend base1 remove new file' > ../logfile
+  $ hg ci --amend -l ../logfile
   saved backup bundle to $TESTTMP/.hg/strip-backup/b8e3cb2b3882-amend-backup.hg (glob)
 
   $ hg cat b