Add start_commit_id to ListLabelHistoryRequest (#81)

To view a diff for a commit on a label we need to be able to get the
previous commit on that label. Added `start_commit_id` so we can fetch
the history starting at the desired commit and using a page size of 2
can also get the previous commit.

buf/registry/module/v1/label_service.proto

@@ -160,6 +160,10 @@ message ListLabelHistoryRequest {
   //
   // If not set, Commits with any CommitCheckStatus value are returned.
   repeated CommitCheckStatus commit_check_statuses = 6 [(buf.validate.field).repeated.items.enum.defined_only = true];
+  // The Commit id to start from.
+  //
+  // It is an error to provide a Commit id that doesn't exist on the Label.
+  string start_commit_id = 7 [(buf.validate.field).string.uuid = true];
 }
 
 message ListLabelHistoryResponse {

buf/registry/module/v1beta1/label_service.proto

@@ -168,6 +168,10 @@ message ListLabelHistoryRequest {
   //
   // If not set, Commits with any CommitCheckStatus value are returned.
   repeated CommitCheckStatus commit_check_statuses = 6 [(buf.validate.field).repeated.items.enum.defined_only = true];
+  // The Commit id to start from.
+  //
+  // It is an error to provide a Commit id that doesn't exist on the Label.
+  string start_commit_id = 7 [(buf.validate.field).string.uuid = true];
 }
 
 message ListLabelHistoryResponse {