Fix handling of empty files in SVN in some cases.
Fix handling of empty files in SVN in some cases.

We ensure that the `base` and `tip` variables are defined before they
are referenced. Static analysis (via PyCharm) determines that the
variables are now always assigned before referencing.

Testing Done:
Ran unit tests.

Before applying this patch, running `rbt diff` in an SVN repo against
two changesets that add an empty file and delete an empty file would
cause rbt to crash with the message:
`CRITICAL: local variable 'base' referenced before assignment`.
After applying this patch, this problem no longer persists. The output
of `rbt diff` has the correct revision numbers in this case.

Reviewed at https://reviews.reviewboard.org/r/7002/

haosdent/rbtools,davidt/rbtools,davidt/rbtools,halvorlu/rbtools,beol/rbtools,beol/rbtools,halvorlu/rbtools,datjwu/rbtools,haosdent/rbtools,davidt/rbtools,beol/rbtools,datjwu/rbtools,haosdent/rbtools,reviewboard/rbtools,halvorlu/rbtools,reviewboard/rbtools,reviewboard/rbtools,datjwu/rbtools