Add --git option to generate output compatible with 'git diff -p'.
This would behave like 'diff -p', except that it would also generate
the extended headers 'old mode', 'new mode', 'deleted file mode', and
'new file mode', and it would quote file names with unusual characters.
GNU patch already parses this format.

Add --include option (opposite of --exclude).

Add SEEK_DATA/SEEK_HOLE sparse file optimization to cmp, diff -q, etc.

Look into sdiff improvement here:
https://web.archive.org/web/20090106164131/http://www.pkix.net/~chuck/sdiff2.diff

Propagate stderr from subprocess so that diff3 does
a better job of explaining _why_:
  > $ diff3 /bin/sh /bin/false /bin/mv
  > diff3: subsidiary program 'diff' failed (exit status 2)
