- reachable from only listed <head>s.
-+
-Note that the explicitly named <head>s are *not* appended to the
-default set of references, but they replace them. In general you
-would want to say `git prune $(git-rev-parse --all) extra1
-extra2` to keep chains of commits leading to extra1, extra2,
-... in addition to what are reachable from your own refs.
-Saying `git prune extra1 extra2` would *lose* objects reachable
-only from the usual refs, which is usually not what you want.
-