Notably, the "--unreachable" flag no longer depends on specified heads,
and we should document what happens if no heads are given.
SYNOPSIS
--------
-'git-fsck-cache' [--tags] [--root] [--delta-depth] [[--unreachable] [--cache] <object>\*]
+'git-fsck-cache' [--tags] [--root] [--delta-depth] [--unreachable] [--cache] [<object>*]
DESCRIPTION
-----------
<object>::
An object to treat as the head of an unreachability trace.
+ If no objects are given, git-fsck-cache defaults to using the
+ index file and all SHA1 references in .git/refs/* as heads.
+
--unreachable::
Print out objects that exist but that aren't readable from any
- of the specified head nodes.
+ of the reference nodes.
--root::
Report root nodes.