fsck: support ignoring objects in `git fsck` via fsck.skiplist

Identical to support in `git receive-pack for the config option
`receive.fsck.skiplist`, we now support ignoring given objects in
`git fsck` via `fsck.skiplist` altogether.

This is extremely handy in case of legacy repositories where it would
cause more pain to change incorrect objects than to live with them
(e.g. a duplicate 'author' line in an early commit object).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -1253,6 +1253,14 @@ that setting `fsck.missingEmail = ignore` will hide that issue.
 This feature is intended to support working with legacy repositories
 which cannot be repaired without disruptive changes.
 
+fsck.skipList::
+	The path to a sorted list of object names (i.e. one SHA-1 per
+	line) that are known to be broken in a non-fatal way and should
+	be ignored. This feature is useful when an established project
+	should be accepted despite early commits containing errors that
+	can be safely ignored such as invalid committer email addresses.
+	Note: corrupt objects cannot be skipped with this setting.
+
 gc.aggressiveDepth::
 	The depth parameter used in the delta compression
 	algorithm used by 'git gc --aggressive'.  This defaults

builtin/fsck.c

@@ -49,6 +49,19 @@ static int show_dangling = 1;
 
 static int fsck_config(const char *var, const char *value, void *cb)
 {
+	if (strcmp(var, "fsck.skiplist") == 0) {
+		const char *path;
+		struct strbuf sb = STRBUF_INIT;
+
+		if (git_config_pathname(&path, var, value))
+			return 1;
+		strbuf_addf(&sb, "skiplist=%s", path);
+		free((char *)path);
+		fsck_set_msg_types(&fsck_obj_options, sb.buf);
+		strbuf_release(&sb);
+		return 0;
+	}
+
 	if (skip_prefix(var, "fsck.", &var)) {
 		fsck_set_msg_type(&fsck_obj_options, var, value);
 		return 0;