filter-branch: fix variable export logic

filter-branch: fix variable export logic

filter-branch tries to restore "old" copies of some
environment variables by using the construct:

  unset var
  test -z "$old_var" || var="$old_var" && export var

This is just wrong.  AND-list and OR-list operators && and || have equal
precedence and they bind left to right.  The second term, var="$old"
assignment always succeeds, so we always end up exporting var.

On bash and dash, exporting an unset variable has no effect. However, on
some shells (such as FreeBSD's /bin/sh), the shell exports the empty
value.

This manifested itself in this case as git-filter-branch setting
GIT_INDEX_FILE to the empty string, which in turn caused its call to
git-read-tree to fail, leaving the working tree pointing at the original
HEAD instead of the rewritten one.

To fix this, we change the short-circuit logic to better match the intent:

  test -z "$old_var" || {
    var="$old_var" && export var
  }

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