classes/metadata_scm: fix git errors showing up on non-git repositories
Fixes the following error showing up for layers that aren't a git repo
(or aren't parented by one):
fatal: Not a git repository (or any of the parent directories): .git
This was because we weren't intercepting stderr. We might as well just
use bb.process.run() here which does that and returns stdout and stderr
separately.
(This was a regression that came in with OE-Core revision
3aac11076e22ac4fea48f5404110bb959547a9fe).
Fixes [YOCTO #8661].
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
