Sped back up fsck, copy --from etc

All commands that often have to read a lot of information from
the git-annex branch should now be nearly as fast as before
the branch was introduced.

Before fsck was taking approximatly 3 hours, now it's running in 8 minutes.

The code is very nasty. It should be rewritten to read the header line
from git cat-file, and then read the specified number of bytes of content.

doc/todo/speed_up_fsck.mdwn

@@ -36,3 +36,5 @@ commands like whereis and add. --[[Joey]]
 > Hmm, except that's actually an error message sent to stderr. Unless
 > stderr is connected to stdout, it might be better to look for a known,
 > empty object. Could just add a git-annex:empty file to that end.
+
+[[done]] --[[Joey]]