Integrate coverage reporting for the test suite
Either by simply calling test-coverage which will use lcov and genhtml
or by running `./configure --enable-gcov`. Here's the current stats:
Hit Total Coverage
---- ----- --------
Lines: 7145 9435 75.7 %
Functions: 602 668 90.1 %
Directory Line Coverage Functions
----------- ------------------- -----------------
compat 63.4 % 234 / 369 60.7 % 17 / 28
include/tig 100.0 % 68 / 68 100.0 % 20 / 20
src 76.0 % 6794 / 8942 91.1 % 561 / 616
test/tools 87.5 % 49 / 56 100.0 % 4 / 4