rev-list: check reflog_info before showing usage

When git-rev-list sees no pending commits, it shows a usage
message. This works even when reflog-walking is requested,
because the reflog-walk code currently puts the reflog tips
into the pending queue.

In preparation for refactoring the reflog-walk code, let's
explicitly check whether we have any reflogs to walk. For
now this is a noop, but the existing reflog tests will make
sure that it kicks in after the refactoring. Likewise, we'll
add a test that "rev-list -g" without specifying any reflogs
continues to fail (so that we know our check does not kick
in too aggressively).

Note that the implementation needs to go into its own
sub-function, as the walk code does not expose its innards
outside of reflog-walk.c.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/rev-list.c

@@ -11,6 +11,7 @@
 #include "graph.h"
 #include "bisect.h"
 #include "progress.h"
+#include "reflog-walk.h"
 
 static const char rev_list_usage[] =
 "git rev-list [OPTION] <commit-id>... [ -- paths... ]\n"
@@ -348,7 +349,7 @@ int cmd_rev_list(int argc, const char **argv, const char *prefix)
 		/* Only --header was specified */
 		revs.commit_format = CMIT_FMT_RAW;
 
-	if ((!revs.commits &&
+	if ((!revs.commits && reflog_walk_empty(revs.reflog_info) &&
 	     (!(revs.tag_objects || revs.tree_objects || revs.blob_objects) &&
 	      !revs.pending.nr)) ||
 	    revs.diff)

reflog-walk.c

@@ -365,3 +365,8 @@ void show_reflog_message(struct reflog_walk_info *reflog_info, int oneline,
 		strbuf_release(&selector);
 	}
 }
+
+int reflog_walk_empty(struct reflog_walk_info *info)
+{
+	return !info || !info->reflogs.nr;
+}

reflog-walk.h

@@ -20,4 +20,6 @@ extern void get_reflog_selector(struct strbuf *sb,
 		const struct date_mode *dmode, int force_date,
 		int shorten);
 
+extern int reflog_walk_empty(struct reflog_walk_info *walk);
+
 #endif

t/t1414-reflog-walk.sh

@@ -102,4 +102,8 @@ test_expect_failure 'walk prefers reflog to ref tip' '
 	test_cmp expect actual
 '
 
+test_expect_success 'rev-list -g complains when there are no reflogs' '
+	test_must_fail git rev-list -g
+'
+
 test_done