Re-implement "check_file_directory_conflict()"

This is (imho) more readable, and is also a lot faster.  The expense of
looking up sub-directory beginnings was killing us on things like
"git-diff-cache", even though that one didn't even care at all about the
file vs directory conflicts.

We really only care when somebody tries to add a conflicting name to
stage 0.

We should go through the conflict rules more carefully some day.

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