* Introduce more sophisticated protocol negotiation for rpush.
* Do not quote `:' in shellquote.
* Test suite: Honour DGIT_TEST_DEBUG=''.
+ * Print a supplementary message when push fails, giving advice to
+ the user about how to retry. Closes:#793144.
+ * Honour dgit-distros.DISTRO.cmd-CMD and .opts-CMD. Closes:#793427.
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