fish: add source_sh function

It's a hack, and has limitations, but works for my day to day use case, with
this I can source the oe-core or meta-mentor setup script, indirectly, from
fish, by sourcing it in sh and re-exec'ing fish.

Signed-off-by: Christopher Larson <kergoth@gmail.com>
bgrm: don't create temp dir if the file doesn't exist

Signed-off-by: Christopher Larson <kergoth@gmail.com>