Merge branch 'pw/checkout-conflict-errorfix'

"git checkout --conflict=bad" reported a bad conflictStyle as if it were given to a configuration variable; it has been corrected to report that the command line option is bad. * pw/checkout-conflict-errorfix: checkout: fix interaction between --conflict and --merge checkout: cleanup --conflict=<style> parsing merge options: add a conflict style member merge-ll: introduce LL_MERGE_OPTIONS_INIT xdiff-interface: refactor parsing of merge.conflictstyle