Version 1.2.1 (2019-02-15)
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  * Support using Sublimerge Pro or Sublimerge 3 as the external
    diff tool by setting `external_diff` to the string
    "sublimerge".

  * Improve detection of an invalid `external_diff` setting that
    could lead to the command being enabled when it should not
    be.

  * Log to the console when the tempoary files created during an
    external diff are removed
