merge-file: honor merge.conflictStyle outside of a repository

merge-file: honor merge.conflictStyle outside of a repository

When running outside a repository, git merge-file ignores the
merge.conflictStyle configuration variable entirely. Since the
function receives `repo` from the caller (which is NULL outside a
repository), and repo_config() falls back to reading system and user
configuration when passed NULL, pass `repo` to repo_config()
unconditionally.

Also document that merge.conflictStyle is honored.

Signed-off-by: Yannik Tausch <dev@ytausch.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>