An object to treat as the head of an unreachability trace.
+
If no objects are given, 'git fsck' defaults to using the
-index file, all SHA1 references in .git/refs/*, and all reflogs (unless
---no-reflogs is given) as heads.
+index file, all SHA1 references in `refs` namespace, and all reflogs
+(unless --no-reflogs is given) as heads.
--unreachable::
Print out objects that exist but that aren't reachable from any