Files in the file list are sorted specially so that header files appear
before their corresponding source files. This change applies this same
sorting algorithm to the comment list.
In order to achieve this, the special file sort function was exposed
publicly -- removed from gr-rest-api-interface and put in a new file,
gr-file-path-behavior.
Bug: Issue 4586
Change-Id: I31079dcef0c3c2421b2561cedf69f68aaf653af4
028d382a5c