tests: Ignore the Test::Harness .prove file

We document how to run prove with the --state option in t/README. This
produces a .prove YAML file in the current directory. Change the t/
gitignore to ignore it, and clean it up on `make clean'.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/.gitignore

@@ -1,2 +1,3 @@
 /trash directory*
 /test-results
+/.prove

t/Makefile

@@ -30,6 +30,7 @@ clean:
 	$(RM) -r 'trash directory'.* test-results
 	$(RM) t????/cvsroot/CVSROOT/?*
 	$(RM) -r valgrind/bin
+	$(RM) .prove
 
 aggregate-results-and-cleanup: $(T)
 	$(MAKE) aggregate-results