trac-svn-hook: fix a test which could fail in non-bash shells and add a note for setting TRAC_ENV directly in the hooks.
trac-svn-hook: fix a test which could fail in non-bash shells and add a note for setting TRAC_ENV directly in the hooks.

pkdevbox/trac,pkdevbox/trac,pkdevbox/trac,pkdevbox/trac