debrebase-last indicate that we are already in gdr format.
* git-debrebase: convert-from-*: leave debrebase-last refs to
hint to everyone that this is now a gdr branch.
+ * git-debrebase: Provide hints for unprocessable commits, depending
+ on the apparent branch ffq state, including possible suggestion
+ to use convert-from-*. Closes:#905005. Closes:#905279.
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