Otherwise for people who use autotools-based configure in main worktree,
the performance testing results will be inconsistent as work and build
trees could be using e.g. different optimization levels.

See e.g.

	http://public-inbox.org/git/20160818175222.bmm3ivjheokf2qzl@sigill.intra.peff.net/

for example.

NOTE config.status has to be copied because otherwise without it the build
would want to run reconfigure this way loosing just copied config.mak.autogen.
Signed-off-by: NKirill Smelkov <kirr@nexedi.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>