Add support for build parameters in Subversion URLs.

Subversion URLs can be parameterized. The plugin should substitute the URL parameters by their default values. Furthermore, the plugin should support URLs ending with @HEAD.

This enhancement request comes from the following comment by Damon Overboe.

The scenario is I wanted to be able to trigger older builds by passing the specific revision, so I added SVN_REVISION as a parameter, defaulting to HEAD.

• Set up a job
• Feature branch it
• Modify the main job to allow SVN_REVISION to be passed in as a parameter,
• and default it to HEAD (may not matter)
• and change your svn url to http://path/to/project/trunk@$SVN_REVISION
• then rebasing a feature will fail, complaining about the path.

Also, if you set up the job initially with the @SVN_REVISION, the plug-in won't be able to find the repo when you go to feature branch it.

The work-around for me is easy enough; rather than specifying the @SVN_REVISION in the URL, I just:

• manually cut a branch from the specific revision I want,
• tell the plug-in to add the feature branch (with the same name)
• Click OK to add the project when it warns me the branch already exists.