libgit2/libgit2 — pull request #2513 by ethomson (2014-08-13 18:58:13+00:00)
Title: Allow NULL error message prefix when class=GITERR_OS

One might be inclined to use the system's error message without a prefix. (Presently, we assume `string` is non-null. Using an empty string will still prefix with the colon, which is sort of ugly.)