add script to checkout all available branches

[bastelfreak]

to make development easier we should add a script that is able to check for all remote branches and make a checkout for them in seperate subdirectories. This allows us to test several things in parallel. What needs to be done:
git rev-parse &>/dev/null to check if we are in a git repo
git remote to check if a remote is configured
git fetch -q -p --all 2>&1 to fetch all branches from remote + delete old local ones
git gc --auto for garbadge collection
now we need to iterate add git branch and check them all out in subrepos

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In the future we can also add another script to autogenerate PXE entries for each branch

[bastelfreak]

this is based on a script from @Bluewind https://paste.xinu.at/m-LDhh/