GitRepo.mergeMsgExists

// write a merge message file and verify it can be read
auto repo = initRepo(_userRepo, OpenBare.yes);
scope(exit) rmdirRecurse(_userRepo);
assert(!repo.mergeMsgExists);

string msgPath = buildPath(repo.path, "MERGE_MSG");
std.file.write(msgPath, "");
assert(repo.mergeMsgExists);
assert(repo.mergeMsg !is null && repo.mergeMsg.length == 0);