Re: [PATCH] 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:1462: convert_to_git_filter_fd: Assertion "ca.drv->clean || ca.drv->process" failed.
>
> This assertion and the one above it are not really necessary, as the
> apply_filter() call bellow them already performs the same checks. And
> when these conditions are 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 both assertions 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.