From Issue #3749, changes in 31aabc4 caused SVN diffs to be generated relative to the checkout root rather than the current working directory.  This led to some regressions and was reverted in 0e8d131.  Add a unit test to attempt to prevent this issue from possibly being reintroduced in the future.

Unit test attempts to generate a diff for a single file by invoking 'svn diff' from three different locations.  This should result in an identical diff for all three test cases.

Ran unit test both before and after 31aabc4 was reverted.  Prior to reversion, test cases 2 and 3 would fail as reported in Issue #3749 causing the unit test to fail.  After reversion, the test cases pass.