functions: handle hostname starting with dash

If a hostname starts with a dash `-` character, the prompt_hostname function
fails because the `string` function interprets it as an option instead
of an argument.

(cherry picked from commit 6c9c033126)

share/functions/prompt_hostname.fish

@@ -1,3 +1,3 @@
 function prompt_hostname --description 'short hostname for the prompt'
-    string replace -r "\..*" "" $hostname
+    string replace -r -- "\..*" "" $hostname
 end