use the same index size for compressed and uncompressed indexes.
Otherwise, decoding the index of a compressed repository requires
significantly more memory.
The tests are now split into individual files for each command. The
separation isn't perfect as many tests make use of multiple commands. In
particular `init`, `backup`, `check` and `list` are used by a larger
number of test cases.
Most tests now reside in files name cmd_<name>_integration_test.go. This
provides a certain indication which commands have significant test
coverage.