sort out Repo API for individual commits & their diffs

[cvrebert]

X-Ref: #56.

I'm not entirely sure having both /commits/<thing>.diff and /commits/<thing> ever worked completely properly with Flask/Werkzeug's routing system. When I started unit testing, some gremlins exposed themselves.

Another alternative is to merge them into a single /commits/<almost-arbitrary-string> endpoint, and then have the view function figure out whether it should serve a diff or JSON, but:

• that guessing logic is annoying to write
• it seems difficult/impossible to have Flask make /commits/<thing>.diff (rather than /commits/<thing>.diff/) and /commits/<thing>/ (rather than /commits/<thing>) be the respective canonical URLs
• and /commit/<thing>.diff is the only version that GitHub recognizes anyway

Therefore, this pull request implements the endpoints GitHub-style (i.e. /commits/<thing>/ and /commit/<thing>.diff). (Yes, their scheme is inconsistent in that all other related APIs are under /commits/*.)

CC: @rajivm @dlindquist @kainswor for review

(P.S.: @rajivm, You seem to have outdone GitHub by managing to support diff-ing the initial commit 😄 )

[cvrebert]

Well, GitHub apparently auto-closed this PR for some reason, but anyway, I'm implementing a rebased version of this as I continue with my merging of develop into master.
Once the code is in place with the test suite passing, I'd be quite open to another PR that adds a working /commits/<refspec>.diff endpoint alias.