[Chat Logs] When migrating file to new name, respect possible new name file

If a new name file exists, copy its contents into our currently tracked
file, because we update our tracked file, the new file can only contain
more recent text (unless a name change back and forth and back happened
since last run, but then the out of order text isn't entirely our fault)

So we copy the new file's text to the bottom of our old file and then
remove the new file, so we can rename our old tracked file to the updated
name.

If we cannot perform the copy, or the delete, we bypass the migration,
leaving our old tracked file as a remnant and begin to track the new name
file because that is more proper than clinging to the old one, and thus
no history is lost to failure.

indra/newview/lllogchat.cpp

@@ -65,6 +65,20 @@ bool LLLogChat::migrateFile(const std::string& old_name, const std::string& file
 {
 	std::string oldfile = makeLogFileNameInternal(old_name);
 	if (!LLFile::isfile(oldfile)) return false; // An old file by this name doesn't exist
+
+	if (LLFile::isfile(filename)) // A file by the new name also exists, but wasn't being tracked yet
+	{
+		auto&& new_untracked_log = llifstream(filename);
+		auto&& tracked_log = llofstream(oldfile, llofstream::out|llofstream::app);
+		// Append new to old and find out if it failed
+		bool failed = !(tracked_log << new_untracked_log.rdbuf());
+		// Close streams
+		new_untracked_log.close();
+		tracked_log.close();
+		if (failed || LLFile::remove(filename)) // Delete the untracked new file so that reclaiming its name won't fail
+			return true; // We failed to remove it or update the old file, let's just use the new file and leave the old one alone
+	}
+
 	LLFile::rename(oldfile, filename); // Move the existing file to the new name
 	return true; // Report success
 }