MergeToRef without Git2Go
This MR swaps out the Git2Go implementation of Merge in UserMergeToRef
with the pure Git implementation. It also gets rid of the AllowConflicts
parameter, since it already defaults to false and after a determination that we no longer need it, gitlab!124239 (merged) is removing it from Rails.
fixes: #5349 (closed)
Edited by John Cai