Suggest full-path refresh() in failure message

This does not suggest or recommend *preferring* to explicitly call
refresh() over the other other techniques, but it clarifies that
the use of refresh() being presented needs a path argument. It also
shows that path in the form of a full path, so users are less
likely to be misled into thinking a command name or other relative
path should be passed to refresh(), which should rarely be done
(since refresh(path) resolves path).

git/cmd.py

@@ -438,7 +438,7 @@ def refresh(cls, path: Union[None, PathLike] = None) -> bool:
                 The git executable must be specified in one of the following ways:
                     - be included in your $PATH
                     - be set via $%s
-                    - explicitly set via git.refresh()
+                    - explicitly set via git.refresh("/full/path/to/git")
                 """
                 )
                 % cls._git_exec_env_var