We often commit snap/xlog files for tests. A snap/xlog file is binary,
but it contains a plain text header and, as a result, may be identified
as a text file by git, which would enable checkpatch checks for it.
Apparently, we don't need to check these files so let's force git to
identify them as binary.

NO_DOC=git
NO_TEST=git
NO_CHANGELOG=git

(cherry picked from commit 24dd505f)