newt: Add external repos patches support

[m-gorecki]

Now if package that adds external repository contains directory patches/external_repo_name, newt will try to apply patches inside this directory to the external repo.

[kasjer]

When patch fails affected git repository stays in state that git am --abort or git am --skip are required. Maybe newt could call git am --abort as not to leave git in the middle of git am session

[m-gorecki]

@kasjer Fix applied

[kasjer]

Verified to work