REPL: make edit_title_case (M-c) call titlecase (#33851)
When edit_title_case was implemented, `uppercasefirst` (née `ucfirst`)
and `titlecase` were doing the same transformation on only one word.
But `titlecase` now transforms non-leading letters to lowercase,
which is more expected for the M-c keyboard combo, as it's the
standard readline behavior (I believe).
