I experience the same problem as in JENKINS-21845 but the suggested workaround is not applicable (because my branch ref is very specific "origin/master").

The failed command looks like "git rev-parse /origin/master". Adding the remote name "origin" in the merge build configuration only ends up to running the command like "git rev-parse origin/origin/master".

It seems the code is joining the Remote and the Ref(Branch) with a "/", but that is incorrect when the ref is "origin/master" (this is recommended when trunk is ambiguous) — the command will become "git rev-parse /origin/master^

".