CliGitAPIImpl.getHeadRev(url) assigned the same SHA1 to every reference

The Map returned by CliGitAPIImpl.getHeadRev(url) contained the
correct keys (the refs), but mistakenly copied the same SHA1 into each
of the values.

Any consumer that was using the values was likely to receive the wrong
value, unless the value they needed was the first SHA1 returned by
"git ls-remote"