doc hash-function-transition: clarify what SHAttered means

Attempt to clarify what the SHAttered attack means in practice for
Git. The previous version of the text made no mention whatsoever of
Git already having a mitigation for this specific attack, which the
SHAttered researchers claim will detect cryptanalytic collision
attacks.

I may have gotten some of the nuances wrong, but as far as I know this
new text accurately summarizes the current situation with SHA-1 in
git. I.e. git doesn't really use SHA-1 anymore, it uses
Hardened-SHA-1 (they just so happen to produce the same outputs
99.99999999999...% of the time).

Thus the previous text was incorrect in asserting that:

    [...]As a result [of SHAttered], SHA-1 cannot be considered
    cryptographically secure any more[...]

That's not the case. We have a mitigation against SHAttered, *however*
we consider it prudent to move to work towards a NewHash should future
vulnerabilities in either SHA-1 or Hardened-SHA-1 emerge.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/hash-function-transition.txt

@@ -28,11 +28,30 @@ advantages:
   address stored content.
 
 Over time some flaws in SHA-1 have been discovered by security
-researchers. https://shattered.io demonstrated a practical SHA-1 hash
-collision. As a result, SHA-1 cannot be considered cryptographically
-secure any more. This impacts the communication of hash values because
-we cannot trust that a given hash value represents the known good
-version of content that the speaker intended.
+researchers. On 23 February 2017 the SHAttered attack
+(https://shattered.io) demonstrated a practical SHA-1 hash collision.
+
+Git v2.13.0 and later subsequently moved to a hardened SHA-1
+implementation by default, which isn't vulnerable to the SHAttered
+attack.
+
+Thus Git has in effect already migrated to a new hash that isn't SHA-1
+and doesn't share its vulnerabilities, its new hash function just
+happens to produce exactly the same output for all known inputs,
+except two PDFs published by the SHAttered researchers, and the new
+implementation (written by those researchers) claims to detect future
+cryptanalytic collision attacks.
+
+Regardless, it's considered prudent to move past any variant of SHA-1
+to a new hash. There's no guarantee that future attacks on SHA-1 won't
+be published in the future, and those attacks may not have viable
+mitigations.
+
+If SHA-1 and its variants were to be truly broken, Git's hash function
+could not be considered cryptographically secure any more. This would
+impact the communication of hash values because we could not trust
+that a given hash value represented the known good version of content
+that the speaker intended.
 
 SHA-1 still possesses the other properties such as fast object lookup
 and safe error checking, but other hash functions are equally suitable