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>
d84c18d989