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* Fix panic in API pulls when headbranch does not exist (#10676)

Backport #10676

* Fix panic in API pulls when headbranch does not exist
* refix other reference to plumbing.ErrReferenceNotFound
Signed-off-by: Andrew Thornton <art27@cantab.net>

* Apply suggestions from code review
Co-Authored-By: Lauris BH <lauris@nix.lv>