buildman: Remove _of_#_ from results directory paths

Currently, the following scenario will rebuild the first commit even
though it is not really necessary - the commit sha or the position in the
patchset did not change:

$ git am <local-patch-0001>
$ tools/buildman/buildman -P -E -W -b master mx6
<do some more development work>
$ git am <local-patch-0002>
$ tools/buildman/buildman -P -E -W -b master mx6 <- will rebuild the first
						    commit as well, even
						    though nothing has
						    changed about it.

This is due to the fact that previous results directories get removed
when the number of commits change. By removing the _of_#_ part of the
directory path, the commits will be rebuilt only if the commit sha or the
position in the patchset changes. Also, update the testcase to reflect this
change.

Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>