UploadPackTest: Move "reachability" tests to its own file

UploadPackTest: Move "reachability" tests to its own file

UploadPackTest is already too long and it is covering too many aspects
of UploadPack. This makes difficult to see what is tests and if all
cases are covered.

Move the reachability-related tests to its own file. This moves also an
auxiliary function, reducing the length of UploadPack. Complete also the
coverage, adding combinations of bitmap availability/commits or
blobs/reachable or not.

Change-Id: Id5cfc9d0118d997da30e3886c91db996a86250fc
Signed-off-by: Ivan Frade <ifrade@google.com>