add clone --shared feature

[enverbisevac]

--shared
When the repository to clone is on the local machine, instead of using hard links, automatically setup .git/objects/info/alternates to share the objects with the source repository. The resulting repository starts out without any object of its own.

[pjbgf]

These changes should work fine given that the current file:// implementation is based on the git CLI. However, this could introduce some challenges if we ever want to do a pure Go implementation of that scheme.

Please also:

• Update COMPATIBILITY.md
• Re-word the commit message to align with the Contributing guidelines
• Add tests to confirm that unsupported schemes (e.g. https, ssh) are being handled correctly

[pjbgf]

@enverbisevac great job! 🥇 thank you for working on this.

[pjbgf]

We are getting Access is denied on Windows:

FAIL: repository_test.go:795: RepositorySuite.TestPlainCloneBareAndShared

repository_test.go:805:
    c.Assert(err, IsNil)
... value *fmt.wrapError = &fmt.wrapError{msg:"failed to add alternate file to git objects dir: Access is denied.", err:0x5} ("failed to add alternate file to git objects dir: Access is denied.")

----------------------------------------------------------------------
FAIL: repository_test.go:823: RepositorySuite.TestPlainCloneShared

repository_test.go:833:
    c.Assert(err, IsNil)
... value *fmt.wrapError = &fmt.wrapError{msg:"failed to add alternate file to git objects dir: Access is denied.", err:0x5} ("failed to add alternate file to git objects dir: Access is denied.")

2023/10/07 17:12:38 comparing mod time: 2023-10-07 17:12:38.[43](https://github.com/go-git/go-git/actions/runs/6441206801/job/17493288695?pr=860#step:6:44)38018 +0000 UTC == 2023-10-07 17:12:38.4338018 +0000 UTC on windows (amd64)
OOPS: 314 passed, 2 skipped, 2 FAILED

@enverbisevac It could be related to the lock on Windows. Do you mind wrapping the errors so that it is easier to find the source of the issue?

[enverbisevac]

@pjbgf Yep error is on Locking:

FAIL: repository_test.go:823: RepositorySuite.TestPlainCloneShared

repository_test.go:833:
    c.Assert(err, IsNil)
... value *fmt.wrapError = &fmt.wrapError{msg:"failed to add alternate file to git objects dir: cannot lock file: Access is denied.", err:(*fmt.wrapError)(0xc00196f600)} ("failed to add alternate file to git objects dir: cannot lock file: Access is denied.")

2023/10/07 20:[50](https://github.com/enverbisevac/go-git/actions/runs/6443391065/job/17495296461#step:6:51):[51](https://github.com/enverbisevac/go-git/actions/runs/6443391065/job/17495296461#step:6:52) comparing mod time: 2023-10-07 20:50:51.116888 +0000 UTC == 2023-10-07 20:50:51.116888 +0000 UTC on windows (amd64)

[pjbgf]

@enverbisevac nice one, thank you for checking. I had a look on go-billy and we have no tests around the locking functionality, so not sure whether there may be an issue there for Windows.

We could disable the locking for Windows for the time being and resolve that in the future. That way we could get the tests to pass and merge this PR. If you are happy with that, please make the necessary adjustments and add a comment to that effect.