Take back some improvements / fixes from SourceWebSVN plugin to SourceSFSVN

This follows discussion on https://github.com/dregad/source-integration/commit/deba255e873879791c339c4efb5e46cef95f2d34#commitcomment-2713308

Changesets https://github.com/mantisbt-plugins/source-integration/commit/deba255e873879791c339c4efb5e46cef95f2d34 and https://github.com/mantisbt-plugins/source-integration/commit/c11a010d491ec7f80fd4b842dc8770098af2fc44 for SourceWebSVN apply *mutatis mutandis* to SourceSFSVN