refactor(git): move the zero-sha check into core/util

lua/git/object.lua

@@ -65,12 +65,6 @@ local function diff_section()
     }
 end
 
----@param sha string?
----@return boolean
-local function is_zero(sha)
-    return sha == nil or sha:match("^0+$") ~= nil
-end
-
 ---@param rev ow.Git.Revision
 ---@return boolean
 local function is_immutable_rev(rev)
@@ -291,7 +285,7 @@ end
 ---@param path string
 ---@return integer?
 local function side_buf(r, blob, path)
-    if not blob or is_zero(blob) then
+    if not blob or util.is_zero_sha(blob) then
         return nil
     end
     local full, status = r:resolve_sha(blob)