The following are utilities that can be used to compare files and folders on mac.
Diffmerge is an excellent utility to compare files and folders on mac. It also allows merge from left to right (only). The interface is nice. Usage is really awesome. Tip: Make sure nothing is select under View menu. This will show only differences in files including subfolders. Once you address that, play with other options to copy missing files etc
Kdiff3 provies a more rudimentary user interface but it does allow powerful merging. It compares file and folder and allows two way merging (really 3 ways). Kdiff actually allows comparing 3 folders which is really awesome.
This is paid software but 15 day trial is available. It is extremely powerful and user friendly. It compares not only text files but images also.