Repository subset support in commit hooks

[mgood]

One of the issues mentioned in #2789 was that the commit hook scripts don't handle the setup where you have one repository containing many projects. You can only have one commit script for the repository, so it needs to determine based on the path of the commit which projects it pertained to.

[cboos]

The new post-commit hook in MultiRepos makes this possible. We should add an example.

[cboos]

I think the new hook (trac-svn-hook) handles this situation correcly.

Let's imaging the following repository structure:

• projA/{trunk,branches,tags}
• projB/{trunk,branches,tags}

Now we have Trac envA using projA as its scope and Trac envB using projB as its scope.

By specifying a TRAC_ENV variable /path/to/envA:/path/to/envB, both environments will be notified of new changesets and changesets "out of scope" will be ignored.