Merge branch 'pw/checkout-conflict-errorfix'

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