oeqa selftest context.py: fix git commands and set branch name

oeqa selftest context.py: fix git commands and set branch name

Check return values of each git command and set initial branch name
to avoid this warning on every test run:

NOTE: Starting bitbake server...
hint: Using 'master' as the name for the initial branch. This default branch name
hint: is subject to change. To configure the initial branch name to use in all
hint: of your new repositories, which will suppress this warning, call:
hint:
hint:   git config --global init.defaultBranch <name>
hint:
hint: Names commonly chosen instead of 'master' are 'main', 'trunk' and
hint: 'development'. The just-created branch can be renamed via this command:
hint:
hint:   git branch -m <name>

Signed-off-by: Mikko Rapeli <mikko.rapeli@linaro.org>