contrib/git-subtree: Use /bin/sh interpreter instead of /bin/bash

contrib/git-subtree: Use /bin/sh interpreter instead of /bin/bash

Use /bin/sh interpreter instead of /bin/bash for contrib/git-subtree:
it's required for systems which don't use bash by default (for example,
FreeBSD), while there seem to be no bashisms in the script (confirmed
by looking through the source and tesing subtree functionality with
FreeBSD's /bin/sh) to require specifically bash and not the generic
posix shell.

Signed-off-by: Dmitry Marakasov <amdmi3@amdmi3.ru>
Signed-off-by: Junio C Hamano <gitster@pobox.com>