git-cherry-pick: add allow-empty option

git cherry-pick fails when picking a non-ff commit that is empty.  The advice
given with the failure is that a git-commit --allow-empty should be issued to
explicitly add the empty commit during the cherry pick.  This option allows a
user to specify before hand that they want to keep the empty commit.  This
eliminates the need to issue both a cherry pick and a commit operation.

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: nhorman

Documentation/git-cherry-pick.txt

@@ -103,6 +103,15 @@ effect to your index in a row.
 	cherry-pick'ed commit, then a fast forward to this commit will
 	be performed.
 
+--allow-empty::
+	By default, cherry-picking an empty commit will fail,
+	indicating that an explicit invocation of `git commit
+	--allow-empty` is required. This option overrides that
+	behavior, allowing empty commits to be preserved automatically
+	in a cherry-pick. Note that when "--ff" is in effect, empty
+	commits that meet the "fast-forward" requirement will be kept
+	even without this option.
+
 --strategy=<strategy>::
 	Use the given merge strategy.  Should only be used once.
 	See the MERGE STRATEGIES section in linkgit:git-merge[1]

builtin/revert.c

@@ -114,12 +114,14 @@ static void parse_args(int argc, const char **argv, struct replay_opts *opts)
 		OPT_END(),
 		OPT_END(),
 		OPT_END(),
+		OPT_END(),
 	};
 
 	if (opts->action == REPLAY_PICK) {
 		struct option cp_extra[] = {
 			OPT_BOOLEAN('x', NULL, &opts->record_origin, "append commit name"),
 			OPT_BOOLEAN(0, "ff", &opts->allow_ff, "allow fast-forward"),
+			OPT_BOOLEAN(0, "allow-empty", &opts->allow_empty, "preserve empty commits"),
 			OPT_END(),
 		};
 		if (parse_options_concat(options, ARRAY_SIZE(options), cp_extra))

sequencer.c

@@ -260,8 +260,8 @@ static int do_recursive_merge(struct commit *base, struct commit *next,
  */
 static int run_git_commit(const char *defmsg, struct replay_opts *opts)
 {
-	/* 6 is max possible length of our args array including NULL */
-	const char *args[6];
+	/* 7 is max possible length of our args array including NULL */
+	const char *args[7];
 	int i = 0;
 
 	args[i++] = "commit";
@@ -272,6 +272,9 @@ static int run_git_commit(const char *defmsg, struct replay_opts *opts)
 		args[i++] = "-F";
 		args[i++] = defmsg;
 	}
+	if (opts->allow_empty)
+		args[i++] = "--allow-empty";
+
 	args[i] = NULL;
 
 	return run_command_v_opt(args, RUN_GIT_CMD);

sequencer.h

@@ -29,6 +29,7 @@ struct replay_opts {
 	int signoff;
 	int allow_ff;
 	int allow_rerere_auto;
+	int allow_empty;
 
 	int mainline;