implemented 'basher upgrade --all'

[pforret]

minimal implementation, although I could delete the
echo "# $package"
if that's too much output.

if I don't put that echo, the output is just a concatenation of git pull outputs, and you can't easily see where one stops and the next one starts

[juanibiapina]

Thanks for your contribution!

Tests are missing, though.

[pforret]

added a test to tests/basher-upgrade.bats

[juanibiapina]

Thanks!