subtree: initialize "prefix" variable

We parse the "--prefix" command-line option into the
"$prefix" shell variable. However, if we do not see such an
option, the variable is left with whatever value it had in
the environment. We should initialize it to a known value,
like we do for other variables.

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

contrib/subtree/git-subtree.sh

@@ -46,6 +46,7 @@ ignore_joins=
 annotate=
 squash=
 message=
+prefix=
 
 debug()
 {