Add __fish_anypython helper function

This just finds the first usable python and echos it, so it can then
be used.

We have a few places where we use it and I'm about to add some more.

share/functions/__fish_anypython.fish

@@ -0,0 +1,10 @@
+function __fish_anypython
+    # Try python3 first, because that's usually faster and generally nicer.
+	for py in python3 python2 python
+        command -sq $py
+        and echo $py
+        and return 0
+    end
+    # We have no python.
+    return 1
+end

share/functions/fish_config.fish

@@ -1,11 +1,7 @@
 function fish_config --description "Launch fish's web based configuration"
     set -lx __fish_bin_dir $__fish_bin_dir
-    if command -sq python3
-        python3 "$__fish_data_dir/tools/web_config/webconfig.py" $argv
-    else if command -sq python2
-        python2 "$__fish_data_dir/tools/web_config/webconfig.py" $argv
-    else if command -sq python
-        python "$__fish_data_dir/tools/web_config/webconfig.py" $argv
+    if set -l python (__fish_anypython)
+        $python "$__fish_data_dir/tools/web_config/webconfig.py" $argv
     else
     	echo (set_color $fish_color_error)Cannot launch the web configuration tool:(set_color normal)
     	echo (set_color -o)fish_config(set_color normal) requires Python.

share/functions/fish_update_completions.fish

@@ -2,12 +2,8 @@ function fish_update_completions --description "Update man-page based completion
     # Don't write .pyc files, use the manpath, clean up old completions
     # display progress.
     set -l update_args -B $__fish_data_dir/tools/create_manpage_completions.py --manpath --cleanup-in ~/.config/fish/generated_completions --progress $argv
-    if command -qs python3
-        python3 $update_args
-    else if command -qs python2
-        python2 $update_args
-    else if command -qs python
-        python $update_args
+    if set -l python (__fish_anypython)
+        $python $update_args
     else
         printf "%s\n" (_ "python executable not found")
         return 1