add git safe directory in action

[musoke]

I have had a strange issue appear when using this action in my repo. The formatter runs successfully, but git complains that it is not in a repo when checking if files have changed. The error occurs on line 132 of format.jl.

Here is an example run that didn't work: https://github.com/musoke/UltraDark.jl/actions/runs/4238135156/jobs/7364871794

And the corresponding configuration:

name: JuliaFormatter

on:
  push:
    branches:
      - 'main'
      - 'release-'
    tags: '*'
  pull_request:

jobs:
  format:
    runs-on: ubuntu-latest
    steps:
    - uses: actions/checkout@master
    - name: ls -a to check for .git
      run: ls -a
    - uses: julia-actions/julia-format@master
      with:
        args: -v .

and the error (snipped a lot of output):

...
Formatting /github/workspace/test/summary.jl
warning: Not a git repository. Use --no-index to compare two paths outside a working tree
usage: git diff --no-index [<options>] <path> <path>

Diff output format options
...
...
ERROR: LoadError: failed process: Process(`git diff --name-only`, ProcessExited(129)) [129]

Stacktrace:
 [1] pipeline_error
   @ ./process.jl:565 [inlined]
 [2] read(cmd::Cmd)
   @ Base ./process.jl:449
 [3] |>(x::Cmd, f::typeof(read))
   @ Base ./operators.jl:911
 [4] top-level scope
   @ //format.jl:132
in expression starting at /format.jl:132

This seems to be because of how directories are nested in the docker environment. See for example actions/checkout#363 (comment)

I changed entrypoint.sh to explicitly mark the directory as safe so git agrees to run. This fixed my CI: https://github.com/musoke/UltraDark.jl/actions/runs/4238608466
I also added a check that git diff --name-only has empty output before the formatter is run.

---

It could well be that I have done something silly.

[hyrodium]

Both entrypoint.sh and format.jl are removed in #24, so this PR can be closed now.

[musoke]

Looking forward to trying it out!