convert: fail gracefully upon missing clean cmd on required filter

convert: fail gracefully upon missing clean cmd on required filter

The gitattributes documentation mentions that either the clean cmd or
the smudge cmd can be left unspecified in a filter definition. However,
when the filter is marked as 'required', the absence of any one of these
two should be treated as an error. Git already fails under these
circumstances, but not always in a pleasant way: omitting a clean cmd in
a required filter triggers an assertion error which leaves the user with
a quite verbose message:

git: convert.c:1459: convert_to_git_filter_fd: Assertion "ca.drv->clean || ca.drv->process" failed.

This assertion is not really necessary, as the apply_filter() call below
it already performs the same check. And when this condition is not met,
the function returns 0, making the caller die() with a much nicer
message. (Also note that die()-ing here is the right behavior as
`would_convert_to_git_filter_fd() == true` is a precondition to use
convert_to_git_filter_fd(), and the former is only true when the filter
is required.) So remove the assertion and add two regression tests to
make sure that git fails nicely when either the smudge or clean command
is missing on a required filter.

Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Junio C Hamano <gitster@pobox.com>