log --grep-reflog: reject the option without -g

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/rev-list-options.txt

@@ -56,8 +56,8 @@ endif::git-rev-list[]
 	Limit the commits output to ones with reflog entries that
 	match the specified pattern (regular expression). With
 	more than one `--grep-reflog`, commits whose reflog message
-	matches any of the given patterns are chosen. Ignored unless
+	matches any of the given patterns are chosen.  It is an
-	`--walk-reflogs` is given.
+	error to use this option unless `--walk-reflogs` is in use.
 
 --grep=<pattern>::
 

grep.c

@@ -64,6 +64,8 @@ void append_header_grep_pattern(struct grep_opt *opt,
 {
 	struct grep_pat *p = create_grep_pat(pat, strlen(pat), "header", 0,
 					     GREP_PATTERN_HEAD, field);
+	if (field == GREP_HEADER_REFLOG)
+		opt->use_reflog_filter = 1;
 	do_append_grep_pat(&opt->header_tail, p);
 }
 

grep.h

@@ -107,6 +107,7 @@ struct grep_opt {
 #define GREP_BINARY_TEXT	2
 	int binary;
 	int extended;
+	int use_reflog_filter;
 	int pcre;
 	int relative;
 	int pathname;

revision.c

@@ -1908,6 +1908,8 @@ int setup_revisions(int argc, const char **argv, struct rev_info *revs, struct s
 
 	if (revs->reflog_info && revs->graph)
 		die("cannot combine --walk-reflogs with --graph");
+	if (!revs->reflog_info && revs->grep_filter.use_reflog_filter)
+		die("cannot use --grep-reflog without --walk-reflogs");
 
 	return left;
 }
@@ -2217,12 +2219,19 @@ static int commit_match(struct commit *commit, struct rev_info *opt)
 	struct strbuf buf = STRBUF_INIT;
 	if (!opt->grep_filter.pattern_list && !opt->grep_filter.header_list)
 		return 1;
-	if (opt->reflog_info) {
+
+	/* Prepend "fake" headers as needed */
+	if (opt->grep_filter.use_reflog_filter) {
 		strbuf_addstr(&buf, "reflog ");
 		get_reflog_message(&buf, opt->reflog_info);
 		strbuf_addch(&buf, '\n');
-		strbuf_addstr(&buf, commit->buffer);
 	}
+
+	/* Copy the commit to temporary if we are using "fake" headers */
+	if (buf.len)
+		strbuf_addstr(&buf, commit->buffer);
+
+	/* Find either in the commit object, or in the temporary */
 	if (buf.len)
 		retval = grep_buffer(&opt->grep_filter, buf.buf, buf.len);
 	else

t/t7810-grep.sh

@@ -572,6 +572,10 @@ test_expect_success 'log grep (9)' '
 	test_cmp expect actual
 '
 
+test_expect_success 'log --grep-reflog can only be used under -g' '
+	test_must_fail git log --grep-reflog="commit: third"
+'
+
 test_expect_success 'log with multiple --grep uses union' '
 	git log --grep=i --grep=r --format=%s >actual &&
 	{