Allow osgeo-post-commit-hook SVN script to receive TRAC name by relative path

[strk]

As trac env location path changed (#1684) all SVN hooks broke (#1882). Letting the osgeo-post-commit-hook SVN script accept relative trac env names would reduce the risk of this happening in the future, and would remove an hard-coded path in the script (which could be replaced by basename $1, already used by some scripts which are likely broken at the moment).