Upgrade go-diff for support Golang 1.16

[blacktear23]

What problem does this PR solve?

After upgrade golang to 1.16 at TiDB make test will got error messages:

../../../../pkg/mod/github.com/pingcap/failpoint@v0.0.0-20200702092429-9f69995143ce/code/restorer.go:24:2: missing go.sum entry for module providing package github.com/sergi/go-diff/diffmatchpatch (imported by github.com/pingcap/failpoint/code); to add:
	go get github.com/pingcap/failpoint/code@v0.0.0-20200702092429-9f69995143ce

What is changed and how it works?

Upgrade go-diff to v1.1.0 will solve this problem.

Check List

Tests

• No Code

Side effects

• Not sure yet, but tests are passed

Related changes

• Need to upgrade go.mod at TiDB and other use failpoint projects

[kennytm]

Lgtm

[bb7133]

lgtm

[bb7133]

PTAL @lonng

[CLAassistant]

All committers have signed the CLA.