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+- cli: pass HEAD by default in argparse
+
+First, make "@" be an alias for HEAD. (Implemented in `get_oid`)
+
+Second, do a little refactoring in *cli.py*. Some commands accept an optional
+OID argument and if the argument isn't provided it defaults to HEAD. For example
+`git log` can get an OID to start logging from, but by default it logs all
+commits before HEAD.
+
+Instead of having each command implement this logic, let's just make "@" (HEAD)
+be the default value for those commands. The relevant commands at this stage
+are `log` and `tag`. More will follow.