Provide the user an option to overwrite existing resolutions using an
`--overwrite` flag. This might be used, for example, if the user knows
that they already have an entry in their rerere cache for a conflict,
but wish to drop it and retrain based on the merge commit(s) passed to
the rerere-train script.
Signed-off-by: Raman Gupta <rocketraman@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>