There are often-used commands (log, diff, checkout come to mind) that can either take commits or file names on the command line, and most disambiguate to commit

There are often-used commands (log, diff, checkout come to mind) that can either take commits or file names on the command line, and most disambiguate to commits. If you introduce . as an alias for HEAD, you either have to change the disambiguation rule, or you must change the meaning of commonly used idioms. Both approaches break backwards compatibility.