Rewrite the diff parser and reduce the memory footprint.

The diff parser code was rewritten for clarity and speed and now handles
a couple of ugly cornercases, like SVN's property change output and single
change chunks, much better. Since the path parsing was unified as well,
the SCM interface gained a new method `getPathStripLevel()` which determines
how many path components need to be shoven off a file name for the SCM
to form a valid path in the workspace (similar to patch(1)'s --strip option).
Fixes issue 627. Automated tests follow.

NEWS.mdtext

@@ -52,6 +52,7 @@
 - Disable browser autocomplete of password fields in the account settings (issue 616)
 - Improvements in the automatic linker parser (issue 618)
 - The `createIssue` API method did not check the API authentication (issue 619)
+- Reduce the memory footprint and compatibility of the internal diff parser (issue 627)
 - Print active git branch heads and tags in bold 
 
 ## Documentation