SubmittingPatches: use generic terms for hash

Since Git is planning on upgrading from SHA-1 to be more hash-agnostic,
replace specific references to SHA-1 with more generic terminology.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/SubmittingPatches

@@ -142,7 +142,7 @@ archive, summarize the relevant points of the discussion.
 
 [[commit-reference]]
 If you want to reference a previous commit in the history of a stable
-branch, use the format "abbreviated sha1 (subject, date)",
+branch, use the format "abbreviated hash (subject, date)",
 with the subject enclosed in a pair of double-quotes, like this:
 
 ....