Merge branch 'disallow-dotgit-via-ntfs-alternate-data-streams'

This patch series plugs an attack vector we had overlooked in our December 2014 work on `core.protectNTFS`. Essentially, the path `.git::$INDEX_ALLOCATION/config` is interpreted as `.git/config` when NTFS Alternate Data Streams are available (which they are on Windows, and at least on network shares that are SMB-mounted on macOS). Needless to say: we don't want that. In fact, we want to stay on the very safe side and not even special-case the `$INDEX_ALLOCATION` stream type: let's just prevent Git from touching _any_ explicitly specified Alternate Data Stream of `.git`. In essence, we'll prevent Git from tracking, or writing to, any path with a segment of the form `.git:<anything>`. Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>