update.sh: reject unknown long opts

I accidentally typed `brew update --greedy` instead of `brew upgrade --greedy` and got some strange output.

This fix causes unrecognized long options to throw a usage message and exit like all the other subcommands.

Before:
```
% brew update --greedy
==> Updating Homebrew...
Usage: brew update-report [--auto-update] [--force]

The Ruby implementation of brew update. Never called manually.
[...]
```

After:
```
% brew update --greedy
Error: Unrecognized option '--greedy'
Usage: brew update, up [options]

Fetch the newest version of Homebrew and all formulae from GitHub using git(1)
and perform any necessary migrations.
[...]
```

Library/Homebrew/cmd/update.sh

@@ -352,7 +352,11 @@ homebrew-update() {
         HOMEBREW_SIMULATE_FROM_CURRENT_BRANCH=1
         ;;
       --auto-update) export HOMEBREW_UPDATE_AUTO=1 ;;
-      --*) ;;
+      --*)
+        onoe "Unknown option: ${option}"
+        brew help update
+        exit 1
+        ;;
       -*)
         [[ "${option}" == *v* ]] && HOMEBREW_VERBOSE=1
         [[ "${option}" == *q* ]] && HOMEBREW_QUIET=1