Always interpret the first word in the command string as the command to
offer completions for, even if that word looks like a flag.
Fixes#3460, where the command string `:-w open` would attempt to offer
completions for `open` but crash because the parsing was thrown off.
By moving the flag-stripping logic to _after_ we determine the command,
`:-w open` interprets `:-w` as the command. Since that is not a valid
command, we won't offer any completions.
c290b3f80f