cli: Flush stdout before fork()ing to run hooks

cli: Flush stdout before fork()ing to run hooks

Without this flush, if stdout is block buffered (which will happen if
it's a pipe or a file, for example) and the hook also writes to
stdout, then notmuch new's output will appear *after* the hook output.

This situation may be a little esoteric, but it's good practice to
flush before you fork anyway.