operations: Provide conflicting commits on merge conflict error

The structured `MergeConflictError` provides information about the files
that have been conflicting, but it doesn't provide any information about
the revisions that caused the merge conflict. While that info should in
the general case already be available on the calling-side, it may not
necessarily be in case the caller e.g. only specified branch names
instead of commit IDs.

Add a new `ConflictingCommitIDs` field to the message that contains the
object IDs of the conflicting commits to provide additional context.

2 files changed, 56 insertions, 40 deletions

diff --git a/proto/errors.proto b/proto/errors.proto
index 59d55cc63..2e4ccd5db 100644
--- a/proto/errors.proto
+++ b/proto/errors.proto
@@ -48,6 +48,9 @@ message MergeConflictError {
   // field is empty, then there has still been a merge conflict, but it wasn't
   // able to determine which files have been conflicting.
   repeated bytes conflicting_files = 1;
+  // ConflictingCommitIds is the set of commit IDs that caused the conflict. In the general case,
+  // this should be set to two commit IDs.
+  repeated string conflicting_commit_ids = 2;
 }
 
 // ReferencesLockedError is an error returned when an ref update fails because
diff --git a/proto/go/gitalypb/errors.pb.go b/proto/go/gitalypb/errors.pb.go
index 8a397eb4a..026bd7985 100644
--- a/proto/go/gitalypb/errors.pb.go
+++ b/proto/go/gitalypb/errors.pb.go
@@ -324,6 +324,9 @@ type MergeConflictError struct {
 	// field is empty, then there has still been a merge conflict, but it wasn't
 	// able to determine which files have been conflicting.
 	ConflictingFiles [][]byte `protobuf:"bytes,1,rep,name=conflicting_files,json=conflictingFiles,proto3" json:"conflicting_files,omitempty"`
+	// ConflictingCommitIds is the set of commit IDs that caused the conflict. In the general case,
+	// this should be set to two commit IDs.
+	ConflictingCommitIds []string `protobuf:"bytes,2,rep,name=conflicting_commit_ids,json=conflictingCommitIds,proto3" json:"conflicting_commit_ids,omitempty"`
 }
 
 func (x *MergeConflictError) Reset() {
@@ -365,6 +368,13 @@ func (x *MergeConflictError) GetConflictingFiles() [][]byte {
 	return nil
 }
 
+func (x *MergeConflictError) GetConflictingCommitIds() []string {
+	if x != nil {
+		return x.ConflictingCommitIds
+	}
+	return nil
+}
+
 // ReferencesLockedError is an error returned when an ref update fails because
 // the references have already been locked by another process.
 type ReferencesLockedError struct {
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 var (