add support for --from-commit and --include-easyblocks-from-commit

[boegel]

WIP because some things are still missing:

• enhanced test for download_repo function to cover new commit option;
• dedicated test for fetch_files_from_commit and fetch_easyconfigs_from_commit functions;
• dedicated test for --from-commit option;
• also implement --include-easyblocks-from-commit option + dedicated test;
• fix combo of --copy-ec and --from-commit

[branfosj]

If I do eb --from-pr=20102 --from-commit=6a7085ddddc63cbfc5fb4f68b522f2bb00de2b22 then the --from-commit is ignored. It would be good to either:

• add a warning message
• fail out on these not being allowed together

[boegel]

If I do eb --from-pr=20102 --from-commit=6a7085ddddc63cbfc5fb4f68b522f2bb00de2b22 then the --from-commit is ignored. It would be good to either:

* add a warning message

* fail out on these not being allowed together

Combination of --*from-commit and --*from-pr is now disallowed, see 6968f87