revisions & diff API: add an "is_dead"

Add an "is_dead" member to "struct diff_options" and "struct rev_info"
as a marker for the diff_free() and release_revisions() functions
already having been run on them.

This will help to ensure that subsequent changes won't introduce the
sort of issues resolved in 5cdb384 (2.36 show regression fix,
2022-04-29) and 91f8f7e (2.36 format-patch regression fix,
2022-04-30).

See
https://lore.kernel.org/git/6af1aed1-ab13-ee0e-e979-d2f826ec776a@web.de
for the initial suggestion to do this.

Suggested-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>

Author: avar

diff.c

@@ -6507,10 +6507,14 @@ static void diff_free_ignore_regex(struct diff_options *options)
 
 void diff_free(struct diff_options *options)
 {
+	if (options->is_dead)
+		BUG("double diff_free() on %p", (void *)options);
+
 	diff_free_file(options);
 	diff_free_ignore_regex(options);
 	clear_pathspec(&options->pathspec);
 	FREE_AND_NULL(options->parseopts);
+	options->is_dead = 1;
 }
 
 void diff_flush(struct diff_options *options)

diff.h

@@ -388,6 +388,8 @@ struct diff_options {
 	struct repository *repo;
 	struct option *parseopts;
 	struct strmap *additional_path_headers;
+
+	int is_dead;
 };
 
 unsigned diff_filter_bit(char status);

revision.c

@@ -3010,6 +3010,9 @@ static void release_revisions_topo_walk_info(struct topo_walk_info *info);
 
 void release_revisions(struct rev_info *revs)
 {
+	if (revs->is_dead)
+		BUG("double release_revisions() on %p", (void *)revs);
+
 	free_commit_list(revs->commits);
 	free_commit_list(revs->ancestry_path_bottoms);
 	object_array_clear(&revs->pending);

revision.h

@@ -338,6 +338,8 @@ struct rev_info {
 
 	/* Location where temporary objects for remerge-diff are written. */
 	struct tmp_objdir *remerge_objdir;
+
+	int is_dead;
 };
 
 /**