subtree: more consistent error propagation

subtree: more consistent error propagation

Ensure that every $(subshell) that calls a function (as opposed to an
external executable) is followed by `|| exit $?`.  Similarly, ensure that
every `cmd | while read; do ... done` loop is followed by `|| exit $?`.

Both of those constructs mean that it can miss `die` calls, and keep
running when it shouldn't.

Signed-off-by: Luke Shumaker <lukeshu@datawire.io>
Signed-off-by: Junio C Hamano <gitster@pobox.com>