rebase -i: mark commits that begin empty in todo editor

While many users who intentionally create empty commits do not want them
thrown away by a rebase, there are third-party tools that generate empty
commits that a user might not want.  In the past, users have used rebase
to get rid of such commits (a side-effect of the fact that the --apply
backend is not currently capable of keeping them).  While such users
could fire up an interactive rebase and just remove the lines
corresponding to empty commits, that might be difficult if the
third-party tool generates many of them.  Simplify this task for users
by marking such lines with a suffix of " # empty" in the todo list.

Suggested-by: Sami Boukortt <sami@boukortt.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-rebase.txt

@@ -620,7 +620,8 @@ commits that started empty, though these are rare in practice.  It
 also drops commits that become empty and has no option for controlling
 this behavior.
 
-The merge backend keeps intentionally empty commits.  Similar to the
+The merge backend keeps intentionally empty commits (though with -i
+they are marked as empty in the todo list editor).  Similar to the
 apply backend, by default the merge backend drops commits that become
 empty unless -i/--interactive is specified (in which case it stops and
 asks the user what to do).  The merge backend also has an

sequencer.c

@@ -4656,6 +4656,9 @@ static int make_script_with_merges(struct pretty_print_context *pp,
 			strbuf_addf(&buf, "%s %s %s", cmd_pick,
 				    oid_to_hex(&commit->object.oid),
 				    oneline.buf);
+			if (is_empty)
+				strbuf_addf(&buf, " %c empty",
+					    comment_line_char);
 
 			FLEX_ALLOC_STR(entry, string, buf.buf);
 			oidcpy(&entry->entry.oid, &commit->object.oid);
@@ -4861,6 +4864,8 @@ int sequencer_make_script(struct repository *r, struct strbuf *out, int argc,
 		strbuf_addf(out, "%s %s ", insn,
 			    oid_to_hex(&commit->object.oid));
 		pretty_print_commit(&pp, commit, out);
+		if (is_empty)
+			strbuf_addf(out, " %c empty", comment_line_char);
 		strbuf_addch(out, '\n');
 	}
 	return 0;