rebase: allow more types of rebases to fast-forward

In the past, we dis-allowed rebases using the interactive backend from
performing a fast-forward to short-circuit the rebase operation.  This
made sense for explicitly interactive rebases and some implicitly
interactive rebases, but certainly became overly stringent when the
merge backend was re-implemented via the interactive backend.

Just as the am-based rebase has always had to disable the fast-forward
based on a variety of conditions or flags (e.g. --signoff, --whitespace,
etc.), we need to do the same but now with a few more options.  However,
continuing to use REBASE_FORCE for tracking this is problematic because
the interactive backend used it for a different purpose.  (When
REBASE_FORCE wasn't set, the interactive backend would not fast-forward
the whole series but would fast-forward individual "pick" commits at the
beginning of the todo list, and then a squash or something would cause
it to start generating new commits.)  So, introduce a new
allow_preemptive_ff flag contained within cmd_rebase() and use it to
track whether we are going to allow a pre-emptive fast-forward that
short-circuits the whole rebase.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/rebase.c

@@ -1467,6 +1467,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 	struct object_id squash_onto;
 	char *squash_onto_name = NULL;
 	int reschedule_failed_exec = -1;
+	int allow_preemptive_ff = 1;
 	struct option builtin_rebase_options[] = {
 		OPT_STRING(0, "onto", &options.onto_name,
 			   N_("revision"),
@@ -1774,13 +1775,20 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 		    state_dir_base, cmd_live_rebase, buf.buf);
 	}
 
+	if ((options.flags & REBASE_INTERACTIVE_EXPLICIT) ||
+	    (action != ACTION_NONE) ||
+	    (exec.nr > 0) ||
+	    options.autosquash) {
+		allow_preemptive_ff = 0;
+	}
+
 	for (i = 0; i < options.git_am_opts.argc; i++) {
 		const char *option = options.git_am_opts.argv[i], *p;
 		if (!strcmp(option, "--committer-date-is-author-date") ||
 		    !strcmp(option, "--ignore-date") ||
 		    !strcmp(option, "--whitespace=fix") ||
 		    !strcmp(option, "--whitespace=strip"))
-			options.flags |= REBASE_FORCE;
+			allow_preemptive_ff = 0;
 		else if (skip_prefix(option, "-C", &p)) {
 			while (*p)
 				if (!isdigit(*(p++)))
@@ -2116,12 +2124,14 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 	/*
 	 * Check if we are already based on onto with linear history,
 	 * in which case we could fast-forward without replacing the commits
-	 * with new commits recreated by replaying their changes. This
-	 * optimization must not be done if this is an interactive rebase.
+	 * with new commits recreated by replaying their changes.
+	 *
+	 * Note that can_fast_forward() initializes merge_base, so we have to
+	 * call it before checking allow_preemptive_ff.
 	 */
 	if (can_fast_forward(options.onto, options.upstream, options.restrict_revision,
 		    &options.orig_head, &merge_base) &&
-	    !is_interactive(&options)) {
+	    allow_preemptive_ff) {
 		int flag;
 
 		if (!(options.flags & REBASE_FORCE)) {

t/t3432-rebase-fast-forward.sh

@@ -30,6 +30,8 @@ test_rebase_same_head () {
 	shift &&
 	test_rebase_same_head_ $status_n $what_n $cmp_n "" "$*" &&
 	test_rebase_same_head_ $status_f $what_f $cmp_f " --no-ff" "$*"
+	test_rebase_same_head_ $status_n $what_n $cmp_n " --merge" "$*" &&
+	test_rebase_same_head_ $status_f $what_f $cmp_f " --merge --no-ff" "$*"
 }
 
 test_rebase_same_head_ () {