bisect: report the found commit with "show"

When "git bisect" finds the first bad commit and shows it to the user,
it calls "git diff-tree" to do so, whose output is meant to be stable
and deliberately ignores end-user customizations.

As the output is supposed to be consumed by humans, replace this with
a call to "git show". This command honors configuration options (such
as "log.date" and "log.mailmap") and other UI improvements (renames
are detected).

Pass some hard-coded options to "git show" to make the output similar
to the one we are replacing, such as showing a patch summary only.

Reported-by: Michael Osipov <michael.osipov@innomotics.com>
Signed-off-By: Peter Krefting <peter@softwolves.pp.se>
Cc: Junio C Hamano <gitster@pobox.com>
Cc: Jeff King <peff@peff.net>
Cc: Christian Couder <christian.couder@gmail.com>

bisect.c

@@ -959,23 +959,30 @@ static enum bisect_error check_good_are_ancestors_of_bad(struct repository *r,
 }
 
 /*
- * This does "git diff-tree --pretty COMMIT" without one fork+exec.
+ * Display a commit summary to the user.
  */
-static void show_diff_tree(struct repository *r,
-			   const char *prefix,
-			   struct commit *commit)
+static void show_commit(struct commit *commit)
 {
-	const char *argv[] = {
-		"diff-tree", "--pretty", "--stat", "--summary", "--cc", NULL
-	};
-	struct rev_info opt;
-
-	git_config(git_diff_ui_config, NULL);
-	repo_init_revisions(r, &opt, prefix);
-
-	setup_revisions(ARRAY_SIZE(argv) - 1, argv, &opt, NULL);
-	log_tree_commit(&opt, commit);
-	release_revisions(&opt);
+	struct child_process show = CHILD_PROCESS_INIT;
+
+	/* Call git show with --no-pager, as it would otherwise
+	 * paginate the "git show" output only, not the output
+	 * from bisect_next_all(); this can be fixed by moving
+	 * it into a --format parameter, but that would override
+	 * the user's default options for "git show", which we
+	 * are trying to honour. */
+	strvec_pushl(&show.args,
+	             "--no-pager",
+	             "show",
+	             "--stat",
+	             "--summary",
+	             "--no-abbrev-commit",
+	             "--diff-merges=first-parent",
+	             oid_to_hex(&commit->object.oid), NULL);
+	show.git_cmd = 1;
+	if (run_command(&show))
+		die(_("unable to start 'show' for object '%s'"),
+		    oid_to_hex(&commit->object.oid));
 }
 
 /*
@@ -1092,7 +1099,7 @@ enum bisect_error bisect_next_all(struct repository *r, const char *prefix)
 		printf("%s is the first %s commit\n", oid_to_hex(bisect_rev),
 			term_bad);
 
-		show_diff_tree(r, prefix, revs.commits->item);
+		show_commit(revs.commits->item);
 		/*
 		 * This means the bisection process succeeded.
 		 * Using BISECT_INTERNAL_SUCCESS_1ST_BAD_FOUND (-10)