The `path_repository` variable is actually confusing to think
about, as it is not always clear what the repository actually is.
It may either be the path to the folder containing worktree and
.git directory, the path to .git itself, a worktree or something
entirely different. Actually, the intent of the variable is to
hold the path to the gitdir, which is either the .git directory
or the bare repository.
Rename the variable to `gitdir` to avoid confusion. While at it,
also rename `path_gitlink` to `gitlink` to improve consistency.
If a branch is already checked out in a working tree we are not
allowed to check out that branch in another repository. Introduce
this restriction when setting a repository's HEAD.
Implement `git_repository_head_for_worktree` and
`git_repository_head_detached_for_worktree` for directly accessing a
worktree's HEAD without opening it as a `git_repository` first.
Implement the `git_worktree_prune` function. This function can be
used to delete working trees from a repository. According to the
flags passed to it, it can either delete the working tree's
gitdir only or both gitdir and the working directory.
Working trees support locking by creating a file `locked` inside
the tree's gitdir with an optional reason inside. Support this
feature by adding functions to get and set the locking status.
Add a new function that checks wether a given `struct
git_worktree` is valid. The validation includes checking if the
gitdir, parent directory and common directory are present.
Introduce a new `struct git_worktree`, which holds information
about a possible working tree connected to a repository.
Introduce functions to allow opening working trees for a
repository.
A repository's configuartion file can always be found in the
GIT_COMMON_DIR, which has been newly introduced. For normal
repositories this does change nothing, but for working trees this
change allows to access the shared configuration file.
The refdb_fs_backend is not aware of the git commondir, which
stores common objects like the o bject database and packed/loose
refereensces when worktrees are used.
Make refdb_fs_backend aware of the common directory by
introducing a new commonpath variable that points to the actual
common path of the database and using it instead of the gitdir
for the mentioned objects.
The commondir variable stores the path to the common directory.
The common directory is used to store objects and references
shared across multiple repositories. A current use case is the
newly introduced `git worktree` feature, which sets up a separate
working copy, where the backing git object store and references
are pointed to by the common directory.
