CMake: Enable testing at the top level again

Doing it in the test/ directory doesn't seem to work
git_fast_import.cpp: Log the first 100 bytes of "data" commands
Make note of the SHA1 for an empty top-level tree

This will come in handy later for branch deletion detection
Improve log readability by emphasizing SVN revision boundaries
PrepareSVNRepo.cmake: correct revision annotations
Run the conversion test in the test subdirectory of the build dir
CMake: Sort out test preparation and add missing dependencies
importer.cpp: handle some files that were being missed; add comments and clarification

We had the logic slightly wrong, causing some important files to be
skipped.  Also, flatten some control-flow nesting and add lots of
comments about what's happening.