feat(api): extend commit and diff services with new functionality

- Add FindCommit, ListCommitsByOid, CommitIsAncestor RPCs to CommitService
- Add CheckObjectsExist, CommitsByMessage, GetCommitStats RPCs to CommitService
- Add LastCommitForPath, CountCommits, CountDivergingCommits RPCs to CommitService
- Add RawDiff, RawPatch, FindChangedPaths RPCs to DiffService
- Add FindMergeBase, WriteRef, SearchFilesByContent RPCs to RepositoryService
- Add SearchFilesByName, ObjectsSize, RepositorySize RPCs to RepositoryService
- Add FindLicense, OptimizeRepository, GetRawChanges RPCs to RepositoryService
- Add FetchRemote, CreateRepositoryFromURL RPCs to RepositoryService
- Implement server handlers for all new RPC methods
- Add new modules for commit counting, finding, and querying features
- Add new modules for diff changed paths and raw operations
- Add new modules for refs and remote operations
- Remove unnecessary comments from various source files
- Update proto definitions with new message types and service methods

proto/remote.proto

@@ -0,0 +1,53 @@
+syntax = "proto3";
+
+package gitks;
+
+import "oid.proto";
+import "repository.proto";
+
+
+message FindRemoteRepositoryRequest {
+  string remote_url = 1;
+}
+
+message RemoteHead {
+  string ref_name = 1;
+  string target_oid = 2;
+  bool symbolic = 3;
+  string symbolic_target = 4;
+}
+
+message FindRemoteRepositoryResponse {
+  repeated RemoteHead refs = 1;
+  bool exists = 2;
+}
+
+message FindRemoteRootRefRequest {
+  string remote_url = 1;
+}
+
+message FindRemoteRootRefResponse {
+  string ref_name = 1;
+  string target_oid = 2;
+}
+
+
+message UpdateRemoteMirrorRequest {
+  RepositoryHeader repository = 1;
+  string remote_url = 2;
+  string remote_name = 3; // defaults to "origin"
+  bool force = 4;
+  bool prune = 5;
+  repeated string refspecs = 6; // if empty, fetch all refs
+}
+
+message UpdateRemoteMirrorResponse {
+  bool ok = 1;
+  string error = 2;
+}
+
+service RemoteService {
+  rpc FindRemoteRepository(FindRemoteRepositoryRequest) returns (FindRemoteRepositoryResponse);
+  rpc FindRemoteRootRef(FindRemoteRootRefRequest) returns (FindRemoteRootRefResponse);
+  rpc UpdateRemoteMirror(UpdateRemoteMirrorRequest) returns (UpdateRemoteMirrorResponse);
+}