Should be able to use "STAGED" as commits in `dolt_commit_diff_[tablename]`

[tbantle22]

I believe this used to work, but getting a diff between STAGED and WORKING using the dolt_commit_diff_[tablename] system table currently fails (dolt v1.39.3).

For example,

SELECT * FROM `dolt_commit_diff_tablename` WHERE `from_commit` = "STAGED" AND `to_commit` = "WORKING"

Returns the error

branch not found: STAGED