download/git: don't require too-recent git

git has supported -C only since 1.8.5, and some distros have not yet
caught up after more than 4 years...

Fall back to entering the directory.

Fixes:
    http://autobuild.buildroot.net/results/35f9f7a4adc6c2cad741079e4afdf1408c94703b



Reported-by: André Hentschel <nerv@dawncrow.de>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Cc: André Hentschel <nerv@dawncrow.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>