buildhistory: fix multiple commit of images and packages at the same time

buildhistory: fix multiple commit of images and packages at the same time

The echo line here was merging multiple lines into one, and the result
was that if both image and package changes had to be comitted then only
the image changes were being committed and the package changes could
potentially be merged into the next package change. Quoting the variable
reference fixes this.

Fixes [YOCTO #2411]

(From OE-Core rev: 2086bb86885951d0a74342e30ff205c24cb93f0d)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>