gitignore: Ignore files generated by "make coverage"

The "make coverage" support added by Thomas Rast in 901c369a didn't
contain a corresponding patch to patch .gitignore.

Change gitignore to ignore the *.gcda, *.gcno and *.gcov files
generated by GCC and our coverage invocations.
Signed-off-by: NÆvar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

.gitignore

@@ -181,6 +181,10 @@
 *.[aos]
 *.py[co]
 .depend/
+*.gcda
+*.gcno
+*.gcov
+/coverage-untested-functions
 *+
 /config.mak
 /autom4te.cache