Provide human friendly message instead of `fatal: --local can only be used inside a git repository`

This run managed to create a .git directory containing a single file (reporter.json):

https://github.com/jsoref/examples-testing/runs/5404530182?check_suite_focus=true#step:4:20

Download action repository 'actions/checkout@v2' (SHA:ec3a7ce113134d7a93b817d10a8272cb61118579)
...
Run actions/checkout@v2
  with:
    repository: jsoref/examples-testing
    token: ***
    ssh-strict: true
    persist-credentials: true
    clean: true
    fetch-depth: 1
    lfs: false
    submodules: false
  env:
    PR_HEAD_SHA: 2f3e6fc9852caf53e728702a7adf0e871e2e1531
    result_code: 
Syncing repository: jsoref/examples-testing
Getting Git version info
/usr/bin/git config --local --get remote.origin.url
Error: fatal: --local can only be used inside a git repository
Deleting the contents of '/home/runner/work/examples-testing/examples-testing'

Which triggers:
https://github.com/actions/checkout/blob/ec3a7ce113134d7a93b817d10a8272cb61118579/src/git-command-manager.ts#L365

The message:

Error: fatal: --local can only be used inside a git repository

... is not particularly helpful to most people.

In this code path, what it means is:

There was a .git directory where the checkout was trying to operate, but it wasn't a valid .git repository.

The next log output line is fine:

Deleting the contents of '/home/runner/work/examples-testing/examples-testing'

Start at src/git-command-manager.ts#L365 and inspect how the failed git config --local --get remote.origin.url call is reported during checkout. Reproduce or trace the invalid .git case described in the issue, then verify that the resulting message explains the invalid repository while the subsequent deletion log remains intact.