Support coverage testing with GCC/gcov

With gcc's --coverage option, we can perform automatic coverage data
collection for the test suite.

Add a new Makefile target 'coverage' that scraps all previous coverage
results, recompiles git with the required compiler/linker flags (in
addition to any flags you specify manually), then runs the test suite
and compiles a report.

The compilation must be done with all optimizations disabled, since
inlined functions (and for line-by-line coverage, also optimized
branches/loops) break coverage tracking.

The tests are run serially (with -j1).  The coverage code should
theoretically allow concurrent access to its data files, but the
author saw random test failures.  Obviously this could be improved.

The report currently consists of a list of functions that were never
executed during the tests, which is written to
'coverage-untested-functions'.  Once this list becomes reasonably
short, we would also want to look at branches that were never taken.

Currently only toplevel *.c files are considered.  It would be nice to
at least include xdiff, but --coverage did not save data to
subdirectories on the system used to write this (gcc 4.3.2).

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -1640,3 +1640,27 @@ check-docs::
 check-builtins::
 	./check-builtins.sh
 
+### Test suite coverage testing
+#
+.PHONY: coverage coverage-clean coverage-build coverage-report
+
+coverage:
+	$(MAKE) coverage-build
+	$(MAKE) coverage-report
+
+coverage-clean:
+	rm -f *.gcda *.gcno
+
+COVERAGE_CFLAGS = $(CFLAGS) -O0 -ftest-coverage -fprofile-arcs
+COVERAGE_LDFLAGS = $(CFLAGS)  -O0 -lgcov
+
+coverage-build: coverage-clean
+	$(MAKE) CFLAGS="$(COVERAGE_CFLAGS)" LDFLAGS="$(COVERAGE_LDFLAGS)" all
+	$(MAKE) CFLAGS="$(COVERAGE_CFLAGS)" LDFLAGS="$(COVERAGE_LDFLAGS)" \
+		-j1 test
+
+coverage-report:
+	gcov -b *.c
+	grep '^function.*called 0 ' *.c.gcov \
+		| sed -e 's/\([^:]*\)\.gcov: *function \([^ ]*\) called.*/\1: \2/' \
+		| tee coverage-untested-functions