Fetch Git commit hash and message for reports

[matejchalk]

Needed for:

• Implement report.md formatting #148

Currently, we only fetch the commit hash, and only for upload. We also need it for formatting reports, where we need the commit message as well for readability.

• replace latestHash function with getLatestCommit which returns other metadata as well as an object
  • sha
  • message
  • author
  • date
• call getLatestCommit function before creating markdown report, pass as parameter to reportToMd function
• for collect: if commit not found (e.g. not run in Git repo), log warning but don't exit - report.md will display commit is unknown
• for upload: if commit not found, throw an error - we can't upload correctly without it