Add support for file browsing and git integration for coveralls

[mfbsouza]

Fixes issue #358.

Description:
This PR adds support for file browsing and git integration when using coveralls with option --coveralls-id=REPO_TOKEN

Details:
For file browsing to work, coveralls expects that the path stored in the "source_files" key it's the same file path for the git repository. To ensure that, this adds the ability to kcov to find the git root path when running it inside of a git project and use it as the strip_path for generating the "source_files" key-pair values.

Git integration was done by adding to the coveralls.out file the expected information defined in coveralls API documentation

Tested with:
What my test repo looks like:

.
├── build.zig
├── build.zig.zon
├── CONTRIBUTING.md
├── kcov-output
│   ├── ...
├── LICENSE
├── README.md
├── shell.nix
├── src
│   ├── memory
│   │   ├── allocator.c
│   │   └── allocator.h
│   └── queue
│       ├── circular_queue.c
│       └── circular_queue.h
├── tests
│   ├── allocator_tests.cpp
│   └── circular_queue_tests.cpp
└── zig-out
    └── bin
        └── boislib_tests

How I ran kcov:
kcov --clean --exclude-pattern=nix,usr,local,cache,_tests.cpp --coveralls-id=MY_REPO_TOKEN kcov-output ./zig-out/bin/boislib_tests

coveralls.out generated:

 cat ./kcov-output/boislib_tests.e67c7dce/coveralls.out
{
 "repo_token": "MY_REPO_TOKEN",
 "git": {
  "head": {
   "id": "9edb5cc7945c5ba3c2055fe896a251d0f5521dfe",
   "author_name": "Matheus 'Bois' Souza",
   "author_email": "mfbsouza.it@gmail.com",
   "committer_name": "GitHub",
   "committer_email": "noreply@github.com",
   "message": "Moving project to use Zig as the build system (#1)"
  },
  "branch": "main"
 },
 "source_files": [
  {
   "name": "src/queue/circular_queue.c",
   "source_digest": "0xc59388d1",
   "coverage": [...]
  },
  {
   "name": "src/memory/allocator.c",
   "source_digest": "0x02902e08",
   "coverage": [...]
  }
 ]
}

What it looks like on coveralls:

[SimonKagstrom]

Nice, thanks!

I'll investigate the ci failures. I don't think they have anything to do with your changes.