abi_check: Allow checking current checkout
abi_check: Allow checking current checkout

Without a "--detach" option, git worktree will refuse to checkout a branch
that's already checked out. This makes the abi_check.py script not very
useful for checking the currently checked out branch, as git will error
that the branch is already checked out. Add the "--detach" option to check
out the new temporary worktree in detached head mode. This is acceptable
because we aren't planning on working on the branch and just want a
checkout to do ABI checking from.

Mbed-TLS/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls