Optimise getting changelogs in airflow-github

[amoghrajesh]

Fix the logic for getting changelog in minor versions so that we get only the changelog that is not present in the RELEASE_NOTES.rst instead of getting everything which is a heavy operation.

How was this tested?

1. Got the result as-is: ./airflow-github changelog origin/v1-8-stable origin/v1-8-test > without-changes
2. Made the changes and then got the results: ./airflow-github changelog origin/v1-8-stable origin/v1-8-test > with-changes
3. Manually verified the content to be same
4. Checked using diff tool: git diff --no-index with-changes without-changes

---

^ Add meaningful description above
Read the Pull Request Guidelines for more information.
In case of fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in a newsfragment file, named {pr_number}.significant.rst or {issue_number}.significant.rst, in newsfragments.

[ephraimbuddy]

Can you test by running it between v2-7-stable and v2-8-stable. Problem was between minors

[amoghrajesh]

Sure @ephraimbuddy, let me check that out and I will post the results here

[amoghrajesh]

@ephraimbuddy there was a bug in the code and I managed to fix it now after some debugging. The issue was that the non core commits were getting included due to faulty else condition.

Managed to test between origin/v2-7-stable and origin/v2-8-stable.

Results:

1. File sizes:

root@8738c81c51bf:/opt/airflow/dev# ls -lh
total 508K
-rw-r--r--  1 root root  29K Jan  3 20:15 2-7-to-2-8-newchanges
-rw-r--r--  1 root root  29K Jan  3 17:20 2-7-to-2-8-original

2. Diff:

root@8738c81c51bf:/opt/airflow/dev# git diff --no-index 2-7-to-2-8-original 2-7-to-2-8-newchanges
diff --git a/2-7-to-2-8-original b/2-7-to-2-8-newchanges
index 78fda81..1862824 100644
--- a/2-7-to-2-8-original
+++ b/2-7-to-2-8-newchanges
@@ -1,4 +1,4 @@
-len of commits 1700
+Number of commits 1700
 Uncategorized
 """""""""""""
 - Update RELEASE_NOTES.rst

The diff is due to different log message, not any actual diff.

[amoghrajesh]

The new approach also saves a hell lot of time actually :)
Realised because this command is super time consuming!

[potiuk]

@ephraimbuddy - WDYT?

[amoghrajesh]

Gentle bump on this one @ephraimbuddy

[amoghrajesh]

Thank you. Sending this one in, since it is a simple fix.

Thanks @potiuk @ephraimbuddy @Lee-W for the reviews