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

tests/test-delta.sh

@@ -135,6 +135,8 @@ rm temp-repo -rf
 echo 'ok generate'
 
 ${CMD_PREFIX} ostree --repo=repo static-delta show ${origrev}-${newrev} > show.txt
+assert_file_has_content show.txt "From: ${origrev}"
+assert_file_has_content show.txt "To: ${newrev}"
 assert_file_has_content show.txt 'Endianness: \(little\|big\)'
 
 echo 'ok show'