[RFC] revision: Don't let ^<rev> cancel out the default <rev>

[Andreas Gruenbacher <agruenba@xxxxxxxxxx>]

Some commands like 'log' default to HEAD if no other revisions are
specified on the command line or otherwise.  Unfortunately, excludes
(^<rev>) cancel out that default, so when a command only excludes
revisions (e.g., 'git log ^origin/master'), the command will not produce
any result.

If all the specified revisions are excludes, it seems more useful to
stick with the default revision instead.

This makes writing wrappers that exclude certain revisions much easier:
for example, a simple alias l='git log ^origin/master' will show the
revisions between origin/master and HEAD by default, and 'l foo' will
show the revisions between origin/master and foo, as you would usually
expect.

Signed-off-by: Andreas Gruenbacher <agruenba@xxxxxxxxxx>
---
 revision.c     | 18 ++++++++++++++----
 t/t4202-log.sh |  6 ++++++
 2 files changed, 20 insertions(+), 4 deletions(-)

diff --git a/revision.c b/revision.c
index de4dce600..c2c51bd5d 100644
--- a/revision.c
+++ b/revision.c
@@ -1158,6 +1158,18 @@ static void add_rev_cmdline(struct rev_info *revs,
 	info->nr++;
 }
 
+static int has_interesting_revisions(struct rev_info *revs)
+{
+	struct rev_cmdline_info *info = &revs->cmdline;
+	unsigned int n;
+
+	for (n = 0; n < info->nr; n++) {
+		if (!(info->rev[n].flags & UNINTERESTING))
+			return 1;
+	}
+	return 0;
+}
+
 static void add_rev_cmdline_list(struct rev_info *revs,
 				 struct commit_list *commit_list,
 				 int whence,
@@ -2318,7 +2330,7 @@ static void NORETURN diagnose_missing_default(const char *def)
  */
 int setup_revisions(int argc, const char **argv, struct rev_info *revs, struct setup_revision_opt *opt)
 {
-	int i, flags, left, seen_dashdash, read_from_stdin, got_rev_arg = 0, revarg_opt;
+	int i, flags, left, seen_dashdash, read_from_stdin, revarg_opt;
 	struct argv_array prune_data = ARGV_ARRAY_INIT;
 	const char *submodule = NULL;
 
@@ -2401,8 +2413,6 @@ int setup_revisions(int argc, const char **argv, struct rev_info *revs, struct s
 			argv_array_pushv(&prune_data, argv + i);
 			break;
 		}
-		else
-			got_rev_arg = 1;
 	}
 
 	if (prune_data.argc) {
@@ -2431,7 +2441,7 @@ int setup_revisions(int argc, const char **argv, struct rev_info *revs, struct s
 		opt->tweak(revs, opt);
 	if (revs->show_merge)
 		prepare_show_merge(revs);
-	if (revs->def && !revs->pending.nr && !revs->rev_input_given && !got_rev_arg) {
+	if (revs->def && !revs->rev_input_given && !has_interesting_revisions(revs)) {
 		struct object_id oid;
 		struct object *object;
 		struct object_context oc;
diff --git a/t/t4202-log.sh b/t/t4202-log.sh
index 153a50615..e6670859c 100755
--- a/t/t4202-log.sh
+++ b/t/t4202-log.sh
@@ -213,6 +213,12 @@ test_expect_success 'git show <commits> leaves list of commits as given' '
 	test_cmp expect actual
 '
 
+printf "sixth\nfifth\n" > expect
+test_expect_success '^<rev>' '
+	git log --pretty="tformat:%s" ^HEAD~2 > actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'setup case sensitivity tests' '
 	echo case >one &&
 	test_tick &&
-- 
2.17.1