commit: copy saved getenv() result

commit: copy saved getenv() result

We save the result of $GIT_INDEX_FILE so that we can restore it after
setting it to a new value and running add--interactive. However, the
pointer returned by getenv() is not guaranteed to be valid after calling
setenv(). This _usually_ works fine, but can fail if libc needs to
reallocate the environment block during the setenv().

Let's just duplicate the string, so we know that it remains valid.

In the long run it may be more robust to teach interactive_add() to take
a set of environment variables to pass along to run-command when it
execs add--interactive. And then we would not have to do this
save/restore dance at all. But this is an easy fix in the meantime.

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