Fix unstaged files affecting the commit message

Can be recreated with the following steps:
```bash
touch file1.txt
echo hi > file2.txt
git add file2.txt
```

file1.txt would be included in the commit message, even though it was
not staged. This removes the unstaged files from the commit message.

Author: spenserblack

commit.go

@@ -30,6 +30,11 @@ func (r *LazyRepo) CommitMsg() (string, error) {
 	if err != nil {
 		return "", err
 	}
+	for filename, fileStatus := range status {
+		if fileStatus.Staging == git.Unmodified || fileStatus.Staging == git.Untracked {
+			delete(status, filename)
+		}
+	}
 
 	if len(status) == 0 {
 		return "", errors.New("no tracked files")