too many spaces indented in *Occur* buffer

[Dan Jacobson]

Gentlemen, there's just too many spaces reserved at the begging of
each line in *Occur* buffers:

40 matches for "q" in buffer: apt-offline.log
     79:Package python-pygresql has broken dep
     80:  Considering python2.3-pygresql 1 as
    115:Package php4-mysql has broken dep on...

We see you have reserved
1234567:
spaces, indicating you normally browse million line files.

OK, no need to dynamically figure out how many spaces. Just reserve 3.
Looks ugly at the 999 1000, 9999 10000 etc. boundary, but let the user
configure it with a variable too.

Wait, M-x grep doesn't cause spaces at all and never looked bad. OK,
make that default 0.