Extract code to print hg root from the prompt

This is useful in other prompts, and potentially also to users.

Don't use a dunderscore because we do way too many of them.

share/functions/__fish_hg_prompt.fish

@@ -27,22 +27,8 @@ function __fish_hg_prompt --description 'Write out the hg prompt'
         return 1
     end
 
-    # Find an hg directory above $PWD
-    # without calling `hg root` because that's too slow
-    set -l root
-    set -l dir (pwd -P)
-    while test $dir != "/"
-        if test -f $dir'/.hg/dirstate'
-            set root $dir"/.hg"
-            break
-        end
-        # Go up one directory
-        set dir (string replace -r '[^/]*/?$' '' $dir)
-    end
-
-    if test -z "$root"
-        return 0
-    end
+    set -l root (fish_print_hg_root)
+    or return 0
 
     # Read branch and bookmark
     set -l branch (cat $root/branch 2>/dev/null; or echo default)

share/functions/fish_print_hg_root.fish

@@ -0,0 +1,22 @@
+function fish_print_hg_root
+    # If hg isn't installed, there's nothing we can do
+    if not command -sq hg
+        return 1
+    end
+
+    # Find an hg directory above $PWD
+    # without calling `hg root` because that's too slow
+    set -l root
+    set -l dir (pwd -P)
+    while test $dir != "/"
+        if test -f $dir'/.hg/dirstate'
+            echo $dir/.hg
+            return 0
+        end
+        # Go up one directory
+        set dir (string replace -r '[^/]*/?$' '' $dir)
+    end
+
+    return 1
+end
+