Add check of main branch commit hash to see if there are any changes to process

[dempseyatgithub]

The extraction tool already compares sha values of individual proposals to detect changes.

The new evolution.json file includes the main branch commit hash used to generate the metadata.

The extraction job should be reworked to check if the main branch has had any changes at all before fetching additional information.

[dempseyatgithub]

Note that even if the swift-evolution repository has the same commit has, the newly generated evolution.json file should not be identical to the original. So you can't just take the data of the old file and write it out as the new file.

At the very least the timestamp must be changed. However, it's also possible that the tool version or schema versions have changed.

Also note that as the number of projects with evolution proposals increases, the tool will need to extract from both subdirectories of /proposals or a directory in another repositories. It would probably be more appropriate to use the SHA of the directory for comparison. This directorySHA or something with a similar name would need to be added to the top level metadata.