Following work in CDRIVER-2505, the root .gitignore should be updated so that all paths correspond to files produced by the current branch.

I ran into an issue where old build artifacts such as test-libmongoc were left around in the repository root despite the most recently built binaries appearing in /src/libmongoc/.

Note: It's possible that could have been avoided by using a shadow build directory; however, the Building from Git documentation did not appear to work correctly on my system as the Makefile was still produced in the repository root.

(From Jesse): The top-level .gitignore also ignores some files that never exist, e.g. we no longer build programs named "repltest1" or "test-mongoc-database". I think those are left over from an ancient implementation of the test suite.