patch-id: make it stable against hunk reordering

Patch id changes if users reorder file diffs that make up a patch.

As the result is functionally equivalent, a different patch id is
surprising to many users.
In particular, reordering files using diff -O is helpful to make patches
more readable (e.g. API header diff before implementation diff).

Add an option to change patch-id behaviour making it stable against
these kinds of patch change:
calculate SHA1 hash for each hunk separately and sum all hashes
(using a symmetrical sum) to get patch id

We use a 20byte sum and not xor - since xor would give 0 output
for patches that have two identical diffs, which isn't all that
unlikely (e.g. append the same line in two places).

The new behaviour is enabled
- when patchid.stable is true
- when --stable flag is present

Using a new flag --unstable or setting patchid.stable to false force
the historical behaviour.

In the documentation, clarify that patch ID can now be a sum of hashes,
not a hash.
Document how command line and config options affect the
behaviour.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
test: add test_write_lines helper

API and implementation as suggested by Junio.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
patch-id-test: test stable and unstable behaviour

Verify that patch ID supports an algorithm
that is stable against diff split and reordering.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
patch-id: change default to stable

--stable has been the default in 'next' for a few weeks with no ill
effects.
Change the default to that so that users don't have to remember to
enable it.

Update documentation to match behaviour change.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
t4204-patch-id.sh: default is now stable

update test to match behaviour change

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>