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
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