rustfmt: markup a TODO item with an issue number

src/prepare.rs

@@ -551,7 +551,7 @@ impl GitWorkArea {
 
         let (name, branch) = if let Some((name, branch_name)) = branch_info {
             if branch_name == "." {
-                // TODO: Pass the branch name we are working on down to here.
+                // TODO(#6): Pass the branch name we are working on down to here.
                 debug!(target: "git.workarea",
                        "the `.` branch specifier for submodules is not supported for conflict \
                         resolution");