Speed up __fish_print_hostnames for faster completions

Thanks to @ThomasAH, as per #4378. Tested on many platforms (OS X,
FreeBSD, Linux, and Solaris). Works with IPv4 and IPv6 as well as
host names and loopback addresses.

share/functions/__fish_print_hostnames.fish

@@ -95,9 +95,24 @@ function __fish_print_hostnames -d "Print a list of known hostnames"
         end
     end
 	for file in $known_hosts
-        # Ignore hosts that are hashed, commented or have custom ports (like [localhost]:2200)
+        if test -r $file
-        test -r $file
+          # Ignore hosts that are hashed, commented or @-marked and strip the key.
-        and string replace -ra '(\S+) .*' '$1' <$file | string match -r '^[^#|[=]+$' | string split ","
+          awk '$1 !~ /[|#@]/ {
+            n=split($1, entries, ",")
+            for (i=1; i<=n; i++) {
+              # Ignore negated/wildcarded hosts.
+              if (!match(entry=entries[i], "[!*?]")) {
+                # Extract hosts with custom port.
+                if (substr(entry, 1, 1) == "[") {
+                  if (pos=match(entry, "]:.*$")) {
+                    entry=substr(entry, 2, pos-2)
+                  }
+                }
+                print entry
+              }
+            }
+          }' $file
+        end
     end
     return 0
 end