merge: add '--continue' option as a synonym for 'git commit'

merge: add '--continue' option as a synonym for 'git commit'

Teach 'git merge' the --continue option which allows 'continuing' a
merge by completing it. The traditional way of completing a merge after
resolving conflicts is to use 'git commit'. Now with commands like 'git
rebase' and 'git cherry-pick' having a '--continue' option adding such
an option to 'git merge' presents a consistent UI.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>