Expose MAX_SIZE via "our" will make it possible
to use in tests, and configure, later.
Additionally, returning HTTP 500 code for big files is not an
Internal Server Error, just a memory limit... Some browsers
won't show our HTML response with the link to the raw file in
case of errors, either, so we'll return 200 to ensure users can
use the link to access the raw blob.
Finally, throw in some tests to the existing solver_git testcase,
since that was incomplete and was pointlessly loading Plack
modules without testing PSGI.