Fix 'additional credentials' help

* Made link visible
* Better documentation for this option
[FIXED JENKINS-18574]

AffectedFile contract specifies "path in the workspace that was affected".

Until now, Subversion plugin returned the full path relative to repo
root instead.

This commit adds storing the relative path to a file in the workspace
to the changelog.xml, and returns that path for AffectedFile.getPath().

Existing Subversion-specific consumers need to switch to getValue()
if they're not using it already, which continues to return the full
path within the repository (e.g. repository browsers).
[JENKINS-18574] Add log parsing test
[JENKINS-22859] Don't fail revprop validation as easily

Only print an error if both of the following are false:
* Field is empty (option 'exclusion revprop' unused)
* Error is authentication failure (there's other fields for that)

The first is a bit shaky due to the global exclusion revprop feature,
but chances are you're not going to see that 'not supported' error
anyway when not explicitly using a per-project setting.
Merge pull request #69 from daniel-beck/fix-help

Fix 'additional credentials' help
Merge pull request #80 from daniel-beck/JENKINS-18574

[RFC] FIX JENKINS-18574/JENKINS-16045
Merge pull request #81 from daniel-beck/JENKINS-22859

[JENKINS-22859] Don't fail revprop validation as easily