Documentation:
* Document the dgit-distro.DISTRO.quilt-mode config setting.
* Clarify the --clean= options' documentation. Closes:#800054.
+ * Discourage use of the --PROGRAM:OPTION escape hatch. (Apropos
+ of various bug reports including #800060 and #833025.)
+ * Document the expected form of HEAD for each --quilt= mode.
Bugfixes:
* When cleaning up after failed clone, stat the to-be-cleaned-up