sample pre-commit hook: use --bool when retrieving config var

Currently if you set

[hooks]
allowNonAscii

(or allownonascii = 1, or = yes) in your .git/config then the sample
pre-commit misinterprets the value as "false" and rejects non-ASCII
filenames.  Use "git config --bool" to get the usual nicer boolean
handling.

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>