sequencer: configurably warn on non-existent files

sequencer: configurably warn on non-existent files

In the future, we plan on externing read_oneliner(). Future users of
read_oneliner() will want the ability to output warnings in the event
that the `path` doesn't exist. Introduce the
`READ_ONELINER_WARN_MISSING` flag which, if active, would issue a
warning when a file doesn't exist by always executing warning_errno()
in the case where strbuf_read_file() fails.

Signed-off-by: Denton Liu <redacted>
Signed-off-by: Junio C Hamano <redacted>