t/Makefile: retain cache t/.prove across prove runs

t/Makefile: retain cache t/.prove across prove runs

prove(1) can write a summary of its test results and timings into a
cache file, t/.prove, then use this information during later runs for
various purposes.  But deleting t/.prove after every test run defeats
this purpose.  So do not delete t/.prove as part of "make
DEFAILT_TEST_TARGET=prove test".  (Continue to delete the file on
"make clean".)

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>