fallback to git last modified upon empty git created date

[weakish]

Description

getGitDate("created", ...) uses git log --diff-filter=A
which returns nothing for files where the latest commit had a merge conflict.
Previously lume would use filesystem last modified date as a fallback.
This commit introduces a change to fall back to git modified first when using special value git created and getGitDate("created", ...) returns nothing.
I think git modified is more reliable than file system creation time under most CI environments.

Related Issues

Check List

• Have you read the
  CODE OF CONDUCT
• Have you read the document
  CONTRIBUTING
  • One pull request per feature. If you want to do more than one thing,
    send multiple pull request.
  • Write tests. (This is for an uncommon corner case so I skipped tests for now.)
  • Run deno fmt to fix the code format before commit.
  • Document any change in the CHANGELOG.md.

[oscarotero]

It looks good to me. Thanks!