test: bisect-porcelain: fix location of files

test: bisect-porcelain: fix location of files

Commit ba7eafe146 (t6030: explicitly test for bisection cleanup,
2017-09-29) introduced checks for files in the $GIT_DIR directory, but
that variable is not always defined, and in this test file it's not.

Therefore these checks always passed regardless of the presence of these
files (unless the user has some /BISECT_LOG file, for some reason).

Let's check the files in the correct location.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>