coverage: do not delete .gcno files before building

The coverage-compile target depends on coverage-clean, which is supposed to remove the earlier build products that would get in the way of the next coverage test run. However, removing *.gcno is actively wrong. These are the files that contain the compile-time coverage related data. They are only rebuilt if the source is compiled. So if one ran 'make coverage' two times in a row, the second run would remove *.gcno, but then fail to recreate them because neither source files nor build flags have changed. (This remained hidden for so long most likely because any other intervening use of 'make' will change the build flags, causing a full rebuild.) So we make an exception for *.gcno. The *.gcda are the coverage results, written when the gcov-instrumented program is run. We still remove those, so as to get a one-test-run view of the data; you could probably argue the other way too. Signed-off-by: Thomas Rast <trast@inf.ethz.ch> Signed-off-by: Junio C Hamano <gitster@pobox.com>