sample_prompts/robbyrussell: Use fish_print_hg_root

This removes ~140ms from every single prompt.

When not in a git repo, this prompt now takes ~9ms, as opposed to
~150ms before.

Fixes #5266 harder.

[ci skip]

share/tools/web_config/sample_prompts/robbyrussell.fish

@@ -34,9 +34,7 @@ function fish_prompt
         end
 
         function _is_hg_repo
-            type -q hg
-            or return 1
-            hg summary >/dev/null 2>/dev/null
+            fish_print_hg_root >/dev/null
         end
 
         function _repo_branch_name