rebase: reinstate --no-keep-empty

Commit d48e5e21da ("rebase (interactive-backend): make --keep-empty the
default", 2020-02-15) turned --keep-empty (for keeping commits which
start empty) into the default.  The logic underpinning that commit was:

  1) 'git commit' errors out on the creation of empty commits without an
     override flag
  2) Once someone determines that the override is worthwhile, it's
     annoying and/or harmful to required them to take extra steps in
     order to keep such commits around (and to repeat such steps with
     every rebase).

While the logic on which the decision was made is sound, the result was
a bit of an overcorrection.  Instead of jumping to having --keep-empty
being the default, it jumped to making --keep-empty the only available
behavior.  There was a simple workaround, though, which was thought to
be good enough at the time.  People could still drop commits which
started empty the same way the could drop any commits: by firing up an
interactive rebase and picking out the commits they didn't want from the
list.  However, there are cases where external tools might create enough
empty commits that picking all of them out is painful.  As such, having
a flag to automatically remove start-empty commits may be beneficial.

Provide users a way to drop commits which start empty using a flag that
existed for years: --no-keep-empty.  Interpret --keep-empty as
countermanding any previous --no-keep-empty, but otherwise leaving
--keep-empty as the default.

This might lead to some slight weirdness since commands like
  git rebase --empty=drop --keep-empty
  git rebase --empty=keep --no-keep-empty
look really weird despite making perfect sense (the first will drop
commits which become empty, but keep commits that started empty; the
second will keep commits which become empty, but drop commits which
started empty).  However, --no-keep-empty was named years ago and we are
predominantly keeping it for backward compatibility; also we suspect it
will only be used rarely since folks already have a simple way to drop
commits they don't want with an interactive rebase.

Reported-by: Bryan Turner <bturner@atlassian.com>
Reported-by: Sami Boukortt <sami@boukortt.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 9 insertions, 6 deletions

diff --git a/builtin/rebase.c b/builtin/rebase.c
index bff53d5d16..022aa2589a 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -85,6 +85,7 @@ struct rebase_options {
 	const char *action;
 	int signoff;
 	int allow_rerere_autoupdate;
+	int keep_empty;
 	int autosquash;
 	char *gpg_sign_opt;
 	int autostash;
@@ -100,6 +101,7 @@ struct rebase_options {
 #define REBASE_OPTIONS_INIT {			  	\
 		.type = REBASE_UNSPECIFIED,	  	\
 		.empty = EMPTY_UNSPECIFIED,	  	\
+		.keep_empty = 1,			\
 		.default_backend = "merge",	  	\
 		.flags = REBASE_NO_QUIET, 		\
 		.git_am_opts = ARGV_ARRAY_INIT,		\
@@ -379,6 +381,7 @@ static int run_sequencer_rebase(struct rebase_options *opts,
 
 	git_config_get_bool("rebase.abbreviatecommands", &abbreviate_commands);
 
+	flags |= opts->keep_empty ? TODO_LIST_KEEP_EMPTY : 0;
 	flags |= abbreviate_commands ? TODO_LIST_ABBREVIATE_CMDS : 0;
 	flags |= opts->rebase_merges ? TODO_LIST_REBASE_MERGES : 0;
 	flags |= opts->rebase_cousins > 0 ? TODO_LIST_REBASE_COUSINS : 0;
@@ -442,6 +445,7 @@ static int run_sequencer_rebase(struct rebase_options *opts,
 	return ret;
 }
 
+static void imply_merge(struct rebase_options *opts, const char *option);
 static int parse_opt_keep_empty(const struct option *opt, const char *arg,
 				int unset)
 {
@@ -449,10 +453,8 @@ static int parse_opt_keep_empty(const struct option *opt, const char *arg,
 
 	BUG_ON_OPT_ARG(arg);
 
-	/*
-	 * If we ever want to remap --keep-empty to --empty=keep, insert:
-	 * 	opts->empty = unset ? EMPTY_UNSPECIFIED : EMPTY_KEEP;
-	 */
+	imply_merge(opts, unset ? "--no-keep-empty" : "--keep-empty");
+	opts->keep_empty = !unset;
 	opts->type = REBASE_MERGE;
 	return 0;
 }
@@ -471,7 +473,7 @@ int cmd_rebase__interactive(int argc, const char **argv, const char *prefix)
 		OPT_NEGBIT(0, "ff", &opts.flags, N_("allow fast-forward"),
 			   REBASE_FORCE),
 		{ OPTION_CALLBACK, 'k', "keep-empty", &options, NULL,
-			N_("(DEPRECATED) keep empty commits"),
+			N_("keep commits which start empty"),
 			PARSE_OPT_NOARG | PARSE_OPT_HIDDEN,
 			parse_opt_keep_empty },
 		OPT_BOOL_F(0, "allow-empty-message", &opts.allow_empty_message,
@@ -1162,6 +1164,7 @@ static int run_specific_rebase(struct rebase_options *opts, enum action action)
 		opts->allow_rerere_autoupdate ?
 			opts->allow_rerere_autoupdate == RERERE_AUTOUPDATE ?
 			"--rerere-autoupdate" : "--no-rerere-autoupdate" : "");
+	add_var(&script_snippet, "keep_empty", opts->keep_empty ? "yes" : "");
 	add_var(&script_snippet, "autosquash", opts->autosquash ? "t" : "");
 	add_var(&script_snippet, "gpg_sign_opt", opts->gpg_sign_opt);
 	add_var(&script_snippet, "cmd", opts->cmd);
@@ -1547,7 +1550,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 			       N_("how to handle commits that become empty"),
 			       PARSE_OPT_NONEG, parse_opt_empty),
 		{ OPTION_CALLBACK, 'k', "keep-empty", &options, NULL,
-			N_("(DEPRECATED) keep empty commits"),
+			N_("keep commits which start empty"),
 			PARSE_OPT_NOARG | PARSE_OPT_HIDDEN,
 			parse_opt_keep_empty },
 		OPT_BOOL(0, "autosquash", &options.autosquash,