rev-list: refactor early option parsing
Before invoking `setup_revisions()`, the `--missing` and
`--exclude-promisor-objects` options are parsed early. In a subsequent
commit, another option is added that must be parsed early.
Refactor the code to parse both options in a single early pass.
Signed-off-by: Justin Tobler <jltobler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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