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>