jjb: increase precision of job searches

The "for `ls .` ... -d subdirectory" logic was examining every entry at
the top-level of the workspace, both files and directories. The files
entries were useless since they would never contain subdirectories.

Use "find" instead to only locate directories at the top level.

The purpose of this change is to make the verbose output (with set -x)
easier to read, and make it easier to spot errors in the future.
Merge pull request #22 from ktdreyer/jjb-better-search

jjb: increase precision of job searches