Merge branch 'en/rebase-backend'

Band-aid fixes for two fallouts from switching the default "rebase"
backend.

* en/rebase-backend:
  git-rebase.txt: highlight backend differences with commit rewording
  sequencer: clear state upon dropping a become-empty commit
  i18n: unmark a message in rebase.c

Documentation/git-rebase.txt

@@ -699,6 +699,16 @@ suffer from the same shortcoming.  (See
 https://lore.kernel.org/git/20200207132152.GC2868@szeder.dev/ for
 details.)
 
+Commit Rewording
+~~~~~~~~~~~~~~~~
+
+When a conflict occurs while rebasing, rebase stops and asks the user
+to resolve.  Since the user may need to make notable changes while
+resolving conflicts, after conflicts are resolved and the user has run
+`git rebase --continue`, the rebase should open an editor and ask the
+user to update the commit message.  The merge backend does this, while
+the apply backend blindly applies the original commit message.
+
 Miscellaneous differences
 ~~~~~~~~~~~~~~~~~~~~~~~~~
 

builtin/rebase.c

@@ -1543,7 +1543,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 				 "ignoring them"),
 			      REBASE_PRESERVE_MERGES, PARSE_OPT_HIDDEN),
 		OPT_RERERE_AUTOUPDATE(&options.allow_rerere_autoupdate),
-		OPT_CALLBACK_F(0, "empty", &options, N_("{drop,keep,ask}"),
+		OPT_CALLBACK_F(0, "empty", &options, "{drop,keep,ask}",
 			       N_("how to handle commits that become empty"),
 			       PARSE_OPT_NONEG, parse_opt_empty),
 		{ OPTION_CALLBACK, 'k', "keep-empty", &options, NULL,

sequencer.c

@@ -1957,6 +1957,8 @@ static int do_pick_commit(struct repository *r,
 		flags |= ALLOW_EMPTY;
 	} else if (allow == 2) {
 		drop_commit = 1;
+		unlink(git_path_cherry_pick_head(r));
+		unlink(git_path_merge_msg(r));
 		fprintf(stderr,
 			_("dropping %s %s -- patch contents already upstream\n"),
 			oid_to_hex(&commit->object.oid), msg.subject);

t/t3424-rebase-empty.sh

@@ -123,4 +123,12 @@ test_expect_success 'rebase --interactive uses default of --empty=ask' '
 	test_cmp expect actual
 '
 
+test_expect_success 'rebase --merge does not leave state laying around' '
+	git checkout -B testing localmods~2 &&
+	git rebase --merge upstream &&
+
+	test_path_is_missing .git/CHERRY_PICK_HEAD &&
+	test_path_is_missing .git/MERGE_MSG
+'
+
 test_done