Give a way to compare a subset of files across repos

[damithc]

In some usage scenarios (e.g., CS2103), it might be useful to compare a file across repositories. That is, the user specifies a glob (e.g., **/TaskList.java) and the code panel shows the code changes to all files matching the glob, across all repositories. This feature can be useful for students to see how others have implemented something, without needing to click through many repos. It might help to catch cases of plagiarism as well (i.e., easy for a viewer to spot two authors implementing something in the exact same 'unusual' way).

One challenge would be to integrate this (rather niche) feature into the current feature set in a general way (i.e., it should not look like 'made for CS2103' feature).