Allow conflicts to be merged in diff

Merged Igor Drozdov requested to merge id-display-conflicts-on-diff into master

What does this MR do?

Related to: #232484 (closed)

gitaly!2626 (merged) introduced a new param to UserMergeToRef RPC which allows us to create merge-ref-head even though there are conflicts. Those conflicts will be merged in the diff with markers and we'll be able to highlight the conflicted block. Something like:

Screen_Shot_2020-10-13_at_21.25.44

Edited by Igor Drozdov