doc hash-function-transition: clarify how older gits die on NewHash

Change the "Repository format extension" to accurately describe what
happens with different versions of Git when they encounter NewHash
repositories, instead of only saying what happens with versions v2.7.0
and later.

See ab9cb76 ("Repository format version check.", 2005-11-25) and
00a09d5 ("introduce "extensions" form of
core.repositoryformatversion", 2015-06-23) for the relevant changes to
the setup code where these variables are checked.

Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: avar

Documentation/technical/hash-function-transition.txt

@@ -116,10 +116,15 @@ Documentation/technical/repository-version.txt) with extensions
 		objectFormat = newhash
 		compatObjectFormat = sha1
 
-Specifying a repository format extension ensures that versions of Git
-not aware of NewHash do not try to operate on these repositories,
-instead producing an error message:
+The combination of setting `core.repositoryFormatVersion=1` and
+populating `extensions.*` ensures that all versions of Git later than
+`v0.99.9l` will die instead of trying to operate on the NewHash
+repository, instead producing an error message.
 
+	# Between v0.99.9l and v2.7.0
+	$ git status
+	fatal: Expected git repo version <= 0, found 1
+	# After v2.7.0
 	$ git status
 	fatal: unknown repository extensions found:
 		objectformat