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 (#1872). 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).

[strk]

For the record: the hooks using relative project name (assuming SVN repo name == trac env name) are currently:

geomoose
ubuntugis
ggl

If I'm right (hadn't checked) those 3 would not be showing in Trac any SVN changes.

[jimk]

While geomoose has history in trac and svn, all active development is on github. So we would not expect any changes in svn or trac.

[jimk]

For clarity:

GeoMoose would like its OSGeo Trac and SVN archived/preserved from a historical perspective. GeoMoose does not expect we will be updating either in the future. (Which makes things like update hooks irrelevant.)

All current development is happening with github (repositories and issues) but history was not migrated when we moved active development from svn to git and from trac to github issues.

[strk]

Moot as nobody should still be using SVN in 2022 :)