extdiff: move single file handling inside `not per-file` conditional If `--per-file` is passed, we will diffing files one by one, hence there is no need to make this optimization around single file in that case. This optimization is only required when `--per-file` is not passed and we will be opening directory-diff for a single file. Differential Revision: https://phab.mercurial-scm.org/D8971