rev-list: Introduce --quiet to avoid /dev/null redirects

Some uses of git-rev-list are to run it with --objects to see if
a range of objects between two or more commits is fully connected
or not.  In such a case the caller doesn't care about the actual
object names or hash hints so formatting this data only for it to
be dumped to /dev/null by a redirect is a waste of CPU time.  If
all the caller needs is the exit status then --quiet can be used
to bypass the commit and object formatting.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

Documentation/git-rev-list.txt

@@ -20,6 +20,7 @@ SYNOPSIS
 	     [ \--not ]
 	     [ \--all ]
 	     [ \--stdin ]
+	     [ \--quiet ]
 	     [ \--topo-order ]
 	     [ \--parents ]
 	     [ \--timestamp ]
@@ -270,6 +271,14 @@ limiting may be applied.
 	In addition to the '<commit>' listed on the command
 	line, read them from the standard input.
 
+--quiet::
+
+	Don't print anything to standard output.  This form of
+	git-rev-list is primarly meant to allow the caller to
+	test the exit status to see if a range of objects is fully
+	connected (or not).  It is faster than redirecting stdout
+	to /dev/null as the output does not have to be formatted.
+
 --cherry-pick::
 
 	Omit any commit that introduces the same change as

builtin-rev-list.c

@@ -26,6 +26,7 @@ static const char rev_list_usage[] =
 "    --remove-empty\n"
 "    --all\n"
 "    --stdin\n"
+"    --quiet\n"
 "  ordering output:\n"
 "    --topo-order\n"
 "    --date-order\n"
@@ -50,6 +51,7 @@ static int show_timestamp;
 static int hdr_termination;
 static const char *header_prefix;
 
+static void finish_commit(struct commit *commit);
 static void show_commit(struct commit *commit)
 {
 	if (show_timestamp)
@@ -93,6 +95,11 @@ static void show_commit(struct commit *commit)
 		strbuf_release(&buf);
 	}
 	maybe_flush_or_die(stdout, "stdout");
+	finish_commit(commit);
+}
+
+static void finish_commit(struct commit *commit)
+{
 	if (commit->parents) {
 		free_commit_list(commit->parents);
 		commit->parents = NULL;
@@ -101,6 +108,12 @@ static void show_commit(struct commit *commit)
 	commit->buffer = NULL;
 }
 
+static void finish_object(struct object_array_entry *p)
+{
+	if (p->item->type == OBJ_BLOB && !has_sha1_file(p->item->sha1))
+		die("missing blob object '%s'", sha1_to_hex(p->item->sha1));
+}
+
 static void show_object(struct object_array_entry *p)
 {
 	/* An object with name "foo\n0000000..." can be used to
@@ -108,9 +121,7 @@ static void show_object(struct object_array_entry *p)
 	 */
 	const char *ep = strchr(p->name, '\n');
 
-	if (p->item->type == OBJ_BLOB && !has_sha1_file(p->item->sha1))
-		die("missing blob object '%s'", sha1_to_hex(p->item->sha1));
-
+	finish_object(p);
 	if (ep) {
 		printf("%s %.*s\n", sha1_to_hex(p->item->sha1),
 		       (int) (ep - p->name),
@@ -527,6 +538,7 @@ int cmd_rev_list(int argc, const char **argv, const char *prefix)
 	int read_from_stdin = 0;
 	int bisect_show_vars = 0;
 	int bisect_find_all = 0;
+	int quiet = 0;
 
 	git_config(git_default_config);
 	init_revisions(&revs, prefix);
@@ -565,6 +577,10 @@ int cmd_rev_list(int argc, const char **argv, const char *prefix)
 			read_revisions_from_stdin(&revs);
 			continue;
 		}
+		if (!strcmp(arg, "--quiet")) {
+			quiet = 1;
+			continue;
+		}
 		usage(rev_list_usage);
 
 	}
@@ -640,7 +656,9 @@ int cmd_rev_list(int argc, const char **argv, const char *prefix)
 		}
 	}
 
-	traverse_commit_list(&revs, show_commit, show_object);
+	traverse_commit_list(&revs,
+		quiet ? finish_commit : show_commit,
+		quiet ? finish_object : show_object);
 
 	return 0;
 }