Currently, diff outputs a file header, even if there are
no changes to the file. This is wonky.
It means that the header chunks are ambiguous, since
not all header chunks are followed by '@@ hunk', and
'--- file', '+++ file' lines can be generated from
file content.
This changes the way that we decide to print the file
header, so we only print it when outputting the first
hunk on flushchanges.
Flushchanges is called once per regular file, at the
end of `diffreg`, so we output a hunk header once per
file.
When diffing non-regular files, like /dev/null,
pipes, and similar, diff will generate a temp
file to diff against. This is the right thing
to do, but the temp file leaks into the diff.
This patch retains the original file name all
the way through to diff output.
The format produced by `diff -u` is inferior to that
produced by `diff -c`, but it's what ape/patch and
unix patch expect, so it's useful to generate it.
This patch adds `diff -u`.
