Get changed files from total diff

[DasSkelett]

Problem

See #66, to get the list of changed files we are currently merging all the changed files of each new commit, even if in the end the file didn't change at all compared to the base branch/ref.

Two examples where this can happen:

1. One commit modifies a file, and another commit reverts it again (e.g. because of PR review)
2. Users "rebasing"/updating their forks with merge commits instead of clean fast-forward merges (Remove KSPIE Incompatibilty NetKAN#8292). Now that merge commit contains all changes from when the last update happened to now, which could be weeks or months worth of CKAN-meta/NetKAN commits.

In both cases, these files actually haven't been changed compared to the base branch (usually current CKAN-meta/NetKAn master). So we don't need to include them in our validation.

Changes

Now instead of iterating over each commit and adding all the changes of the diff of each commit together, we get the total/final diff and extract the changed files from that one.

I've tested this using the following PoC code:
test.py:

from git import Repo

print('Old:')
changed_files = list()
for diffIndex in [commit.parents[0].diff(commit) for commit in Repo('.').iter_commits('upstream/master..HEAD')]:
    for diff in diffIndex.iter_change_type('A'):
        changed_files.append(diff.b_path)
print(changed_files) 

print('New:')
diffIndex = Repo('.').commit('upstream/master').diff('HEAD')
changed_files = [ch.b_path for ch in diffIndex.iter_change_type('A')]
print(changed_files)

Terminal:

cd NetKAN/
git reset --hard HEAD~5
# Create a merge commit similar to https://github.com/KSP-CKAN/NetKAN/pull/8292 :
git merge --no-ff upstream/master
python3 test.py

Output:

Old:
['.github/workflows/inflate.yml', 'NetKAN/KiwiTechTree.netkan', 'NetKAN/TestLite.netkan']
New:
[]

Fixes #66

[DasSkelett]

Wanted to test this with a PR in my NetKAN fork (DasSkelett/NetKAN#2), but this isn't as easy anymore since KSP-CKAN/CKAN@ad3ccbe / 6f96e28, hang on...

Edit: got it to run using another branch with a revert of 6f96e28: https://github.com/DasSkelett/NetKAN/pull/2/checks?check_run_id=1628906987
Looks like it does work.

[HebaruSan]

Thanks!

[HebaruSan]

Awesome.
https://github.com/KSP-CKAN/NetKAN/runs/1641738991?check_suite_focus=true

[techman83]

Brilliant!