CLI: make variable n_requested_db_uuid file scope.

CLI: make variable n_requested_db_uuid file scope.

It turns out that now that we pass an open database into the
subcommands, it is easy to check any requested uuid against the
database at the same time as we process the other shared
arguments. This results in overall less boilerplate code, as well as
making a CLI scope function and variable file scope in notmuch.c.