Makefile: add coverage-prove target

Makefile: add coverage-prove target

Sometimes there are test failures in the 'pu' branch. This
is somewhat expected for a branch that takes the very latest
topics under development, and those sometimes have semantic
conflicts that only show up during test runs. This also can
happen when running the test suite with different GIT_TEST_*
environment variables that interact in unexpected ways

This causes a problem for the test coverage reports, as
the typical 'make coverage-test coverage-report' run halts
at the first failed test. If that test is early in the
suite, then many valuable tests are not exercising the code
and the coverage report becomes noisy with false positives.

Add a new 'coverage-prove' target to the Makefile,
modeled after the 'coverage-test' target. This compiles
the source using the coverage flags, then runs the test
suite using the 'prove' tool. Since the coverage
machinery is not thread-safe, enforce that the tests
are run in sequence by appending '-j1' to GIT_PROVE_OPTS.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>