magit-read-rev: raise error if user makes no selection

Do so by calling `magit-completing-read' with non-nil REQUIRE-MATCH.

There was only one caller for which getting nil back from
`magit-read-rev' made sense.  Reading the end rev in
`magit-read-rev-range' has been changed to not use `magit-read-rev'
to read the end rev.

There were several callers who did explicitly make sure the value
returned by `magit-read-rev' was non-nil before doing anything.
These asserts are no unnecessary and have been removed.  The fact
that such checks existed until now shows that people did run into
this problem and "fixed" it in the particular instance the noticed
without addressing the real issue; that has been taken care of now.
cleanup magit-ido-completing-read and magit-read-file-from-rev
magit-read-remote: make argument PROMPT mandatory