Fix some bugs in ledger-complete-at-point date completion

[bcc32]

ledger-complete-at-point currently seems completely broken for date completion (e.g., "19" completes to the 19th of the current month, or the previous month, whichever date is in the past from today).

I added some regression tests that demonstrate the bugs, and then separate commits to fix each bug. I plan to squash the test and fix commits together when merging.

(@purcell, I spent some time fighting with the CI over some old calling conventions for functions (func-arity, which didn't exist; and encode-time, which doesn't support a single argument in Emacs < 27, but the multiple-argument form is marked "obsolete" in Emacs 29). What's ledger-mode's policy on supporting Emacs major versions, is there generally some guideline we follow as to when we're okay with abandoning support for older versions?)

Fix #419.