builtin/commit.c: remove the PATH_MAX limitation via dynamic allocation

builtin/commit.c: remove the PATH_MAX limitation via dynamic allocation

Remove the PATH_MAX limitation from the environment setting that
points to a filename by switching to dynamic allocation.

As a side effect of this change, we also reduce the snprintf()
calls, that may silently truncate results if the programmer is not
careful.

Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Elia Pinto <gitter.spiros@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>