Include message file in error resolution

When a command has an error subrepo prints instructions on how to
resolve the issue. If the user supplied a commit message file that
information was not included in the resolution information.

lib/git-subrepo

@@ -1808,9 +1808,15 @@ This is the common conflict resolution workflow:
   7. git subrepo push $subdir $branch_name
 ...
   else
+    if [[ $commit_msg_file ]]; then
+    cat <<...
+  7. git subrepo commit --file=$commit_msg_file $subdir
+...
+    else
     cat <<...
   7. git subrepo commit $subdir
 ...
+    fi
   fi
 
   if [[ $command == pull && $join_method == rebase ]]; then