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Git creates `.git/commondir` when there are custom worktrees (see "git worktree add" related commands).
`.git/commondir` in such case contains a link to another dot-git repository tree, which could contain some folders like:
- objects;
- config;
- refs;
- etc.
In this PR a new dotgit.RepositoryFilesystem struct is defined, which is billy.Filesystem interface compatible object-wrapper, that can handle commondir and dispatch all operations to the correct file path.
`git.PlainOpen` remain unchanged, but `git.PlainOpenWithOptions` has a new option: `PlainOpenOptions.EnableDotGitCommonDir=true|false` (which is false by default). When `EnableDotGitCommonDir=true` repository-open procedure will read `.git/commondir` (if it exists) and then create dotgit.RepositoryFilesystem object initialized with 2 filesystems. This object then passed into storage and then into dotgit.DotGit as `billy.Filesystem` interface. This object will catch all filesystem operations and dispatch to the correct repository-filesystem (dot-git or common-dot-git) according to the rules described in the doc: https://git-scm.com/docs/gitrepository-layout#Documentation/gitrepository-layout.txt. EnableDotGitCommonDir option will only work with the filesystem-backed storage.
Also worktree_test.go has been adopted from an older, already existing existing PR: https://github.com/src-d/go-git/pull/1098. This PR needs new fixtures added in the following PR: https://github.com/go-git/go-git-fixtures/pull/1.
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