Update files

Author: apstndb

README.md

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+A copy of `golang.org/x/tools/internal/diff`.
+
+This repo is extracted from https://github.com/golang/tools.
+
+```sh
+git filter-repo --path LICENSE --path go.mod --path go.sum --path internal/diff --path internal/testenv --path internal/gocommand --path internal/goroot --path internal/event --path-rename internal/diff:diff --replace-text ../rename-text.txt
+```

replace-text.txt

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+golang.org/x/tools/internal/diff==>github.com/apstndb/gotoolsdiff/diff
+golang.org/x/tools/internal/testenv==>github.com/apstndb/gotoolsdiff/internal/testenv
+golang.org/x/tools/internal/goroot==>github.com/apstndb/gotoolsdiff/internal/goroot
+golang.org/x/tools/internal/gocommand==>github.com/apstndb/gotoolsdiff/internal/gocommand
+golang.org/x/tools/internal/event==>github.com/apstndb/gotoolsdiff/internal/event
+golang.org/x/tools==>github.com/apstndb/gotoolsdiff