externalsrc: avoid race in temporary git index file
Use a unique tempfile as the temporary git index file when determining
the git hash of the source tree. A fixed filename was obviously causing
races (with the git index.lock file) under oe-selftest where multiple
bitbake instances were run against the same source tree at the same
time.
Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
