tgit.py: set BUP_MAIN_EXE and BUP_DIR in testpacks().

Set these variables in testpacks(), just as we do for
test_pack_name_lookup(), since it also depends on path.exe().

This was causing 'make check' to fail on Cygwin, and could be
reproduced on Linux by explicitly invoking tgit.py via "./wvtest.py
lib/bup/t/tgit.py".

Reported-by: Will Rouesnel <w.rouesnel@gmail.com>
Signed-off-by: Rob Browning <rlb@defaultvalue.org>