gitpkgv.bbclass: cache GITPKGV result

gitpkgv runs the 'git rev-list | wc -l' several times when processing a
package using GITPKGV.  This takes ages for packages like the linux
kernel which has a) a large repository and b) lots of subpackages.

This patch caches the result of 'git rev-list' and uses it on the next
run.

Signed-off-by: Enrico Scholz <enrico.scholz@sigma-chemnitz.de>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>