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6017: Don't return any TextEdit if formatting is unchanged r=jonas-schievink a=cuviper I found that `textDocument/formatting` was always returning a full `TextEdit` replacement, even when there are no changes, which caused Vim (w/ vim-lsp) to always indicate a modified buffer after formatting. We can easily compare whether there were changes and return `null` if not, so the client knows there's nothing to do. Co-authored-by: Josh Stone <cuviper@gmail.com>