An intermediate commit whose net effect is already on HEAD (redundant
change, or 3-way merged to HEAD by "theirs == ours") stopped the
sequencer with "The previous cherry-pick is now empty" and was
treated as a hard conflict. mergeTaskBranches aborted the whole range,
marked the branch failed, and dropped every remaining non-overlapping
commit.
Add cherryPick.skip and cherryPick.isEmptyError to the git namespace,
then in mergeTaskBranches' catch classify the failure before aborting:
loop --skip while the error stderr matches the "now empty" phrase so
consecutive empties advance the sequencer; fall through to abort/fail
on the first non-empty error (genuine conflict with unmerged files).
Fixes#4438
roboomp
12acf7e645