Re: [PATCH 2/2] xdiff: implement empty line chunk heuristic

Jeff King [off-list ref] writes:

I mean that if you save any old patch-ids from "git patch-id", they
won't match up when compared with new versions of git. We can probably
ignore it, though. This isn't the first time that patch-ids might have
changed, and I think the advice is already that one should not count on
them to be stable in the long term.

Another thing that this *will* break is the patch signature upload
protocol k.org uses to allow Linus, Greg, et al. on the road with
limited hotel wifi bandwidth to prepare patch-X-test1.gz and
patch-X-test1.sign file.  They can locally tag X-test1, prepare
"git diff X X-test1 | gzip -n >patch-X-test1.gz" and sign the
result, and upload _only_ the detached signature after pushing.

They can tell k.org, when uploading the detached signature, to
recreate the patchfile by running the same "git diff" to save the
bandwidth of sending the same thing twice (as they have to "push"
anyway, having to send the generated patch is a pure overhead).

Having said all that, kup(1) users are already warned that the
textual diff produced by "git diff-tree -p" (which is mentioned in
the documentation of the tool) varies across versions of Git and
the above "optimization" would not work unless both ends have the
same version of Git, so it may not be too big an issue for them.
They have already been burned once when we corrected "git archive"
output in the past (they obviously have the same optimization to
sign tarballs, and the kup(1) mechanism relies to have byte-for-byte
identical output).