git clone: fall back to native git binary

On Debian 8, using git from the Alpine chroot does not work properly. With this commit, pmbootstrap will fall back to the native git binary for the clone.

Details

• new function pmb.helpers.git.clone_outside()
• add docstrings to pmb.helpers.git.clone() and clone_outside()
• show the git output to the terminal in both functions, so the user better knows what's going on

How to test

• Either run pmbootstrap on Debian 8
• Or:
  • Clone as usually
  • Install git, but delete the binary: pmbootstrap chroot --add=git -- rm /usr/bin/git
  • Remove or rename $work/cache_git/pmaports
  • Run pmbootstrap init, it should fail to use the git from the chroot and fall back to the native one.

Fixes #1662 (closed).