Verify fix for bogus reports of line ending changes in a clean git checkout

[mattmccutchen-cci]

Sometimes, git shows source files in a clean checkout as having modified line endings, even if you only ever use Linux. The problem seems to occur less often with newer versions of git. We believe the root cause is that the root .gitattributes file inaccurately states that all files have LF endings, but some have CRLF endings in the repository. We should fix this in Microsoft's or even LLVM's repository if possible, or otherwise in ours after it diverges from Microsoft (#302).

[mattmccutchen-cci]

This problem was only ever at the Microsoft level, not LLVM. Microsoft has recently merged checkedc#950, which I fully expect to fix it, but I want to do some more testing before I update our documentation.