wrap-for-bin: do not export an empty GIT_TEMPLATE_DIR
wrap-for-bin: do not export an empty GIT_TEMPLATE_DIR

With bash on some platforms (e.g. FreeBSD 8.0), exporting an unset
variable does not "unexport" it.  The called process gets an empty
string from getenv(3) instead of NULL.

Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git