Jeff King authored Jun 24, 2014 and Junio C Hamano committed Jun 25, 2014

"Check the value of an environment and fall back to a known path
inside $GIT_DIR" is repeated a few times to determine the location
of the data store, the index and the graft file, but the return
value of getenv is not guaranteed to survive across further
invocations of setenv or even getenv.

Make sure to xstrdup() the value we receive from getenv(3), and
encapsulate the pattern into a helper function.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>