they can be disambiguated by placing `--` between them.
E.g. `git diff -- HEAD` is, "I have a file called HEAD in my work
tree. Please show changes between the version I staged in the index
- and what I have in the work tree for that file". not "show difference
+ and what I have in the work tree for that file", not "show difference
between the HEAD commit and the work tree as a whole". You can say
`git diff HEAD --` to ask for the latter.