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>