checkout: cleanup --conflict=<style> parsing

checkout: cleanup --conflict=<style> parsing

Passing an invalid conflict style name such as "--conflict=bad" gives
the error message

    error: unknown style 'bad' given for 'merge.conflictstyle'

which is unfortunate as it talks about a config setting rather than
the option given on the command line. This happens because the
implementation calls git_xmerge_config() to set the conflict style
using the value given on the command line. Use the newly added
parse_conflict_style_name() instead and pass the value down the call
chain to override the config setting. This also means we can avoid
setting up a struct config_context required for calling
git_xmerge_config().

The option is now parsed in a callback to avoid having to store the
option name. This is a change in behavior as now

    git checkout --conflict=bad --conflict=diff3

will error out when parsing "--conflict=bad" whereas before this change
it would succeed because it would only try to parse the value of the
last "--conflict" option given on the command line.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>