Unified diff format is not supported for external diffs

[gregorias]

• nvim --version: NVIM v0.5.0-dev+1302-g8a93d1028

Steps to reproduce using nvim -u NORC

" s.vim
function MyDiff()
  silent execute "!diff  -u " . v:fname_in . " " . v:fname_new . " > " . v:fname_out
endfunction
set diffexpr=MyDiff()

# Steps to reproduce
nvim -u NORC -O a b
# Inside Neovim:
:source s.vim
:windo diffthis

Actual behaviour

Vim outputs E97: Cannot create diffs

Expected behaviour

Vim turns on the diff mode without producing an error.

Notes

This is a bug, because documentation claims that Neovim should handle unified diffs (https://neovim.io/doc/user/diff.html, C^F "unified").

Neovim does handle unified diffs for internal diffs since cf1ffa9, but it seems that this functionality has not been plugged into external diffs, and there's even a smoke check that tests that the external diff complies with the ed-style format:

neovim/src/nvim/diff.c

Lines 985 to 992 in 1186f7d

	// There must be a line that contains "1c1".
	if (vim_fgets(linebuf, LBUFLEN, fd)) {
	break;
	}
	if (STRNCMP(linebuf, "1c1", 3) == 0) {
	ok = kTrue;
	}

[gregorias]

Thank you, @glacambre, for preparing a commit that solves this issue.

My intention is to go a step further and refactor the diff.c code a bit. In particular I intend to extract the diff parsing code from diff_read. Right now diff parsing has at least two following code smells:

1. Diff parsing is entangled with updating buffer specific diff structures in diff_read.
2. Parsing logic is in two places check_external_diff and diff_read.

A better structure would be to have one generic diff_parse that parses unified and and ed-style diffs into a generic diff structure, and check_external_diff and diff_read rely on diff_parse. It should make the code easier to read, and future-proof against adding another diff format.

[gregorias]

In #14614 I've extracted diff_parse so that it can be shared across check_external_diff and diff_read. This way external diffs will always be able to use the same formatting that internal diffs can.

[lewis6991]

vim-patch 8.2.2880 was merged in 6d932cc

Does that fix this issue?