fix: make certain git-credential helper implementations work from pro…

…mpts on Windows (#1103)

On prompts, typically only `git.exe` is available, but no shell (i.e. `sh`), wo we have
to prefer going through `git.exe` to launch credential helpers.

gix-credentials/src/program/mod.rs

@@ -65,6 +65,11 @@ impl Program {
         }
         from_custom_definition_inner(input.into())
     }
+
+    /// Convert the program into the respective command, suitable to invoke `action`.
+    pub fn to_command(&self, action: &helper::Action) -> std::process::Command {
+        todo!("fix")
+    }
 }
 
 /// Builder