static-delta: Change `show` to display from/to commits

static-delta: Change `show` to display from/to commits

When writing a delta to a file this may not always be recorded
in the filename, and it's useful data.

Ref: https://mail.gnome.org/archives/ostree-list/2019-February/msg00000.html

This also required teaching `show` to accept a file path.

Note...for some reason `test-deltas.sh` breaks when run from
a tty - we get `SIGTTIN` which implies something is reading from
the tty but it wasn't obvious to me what.

Closes: #1823
Approved by: jlebon