refdb: catch additional per-worktree refs

Minor comment fix

The upstream git.git project currently identifies all references inside
of `refs/bisect/` as well as `HEAD` as per-worktree references. This is
already incorrect and is currently being fixed by an in-flight topic
[1]. The new behavior will be to match all pseudo-references outside of
the `refs/` hierarchy as well as `refs/bisect/`.

Our current behavior is to mark a selection of pseudo-references as
per-worktree, only. This matches more pseudo-references than current
git, but forgets about `refs/bisect/`. Adjust behavior to match the
in-flight topic, that is classify the following references as
per-worktree:

- everything outside of `refs/`
- everything inside of `refs/bisect/`

[1]: <20170213152011.12050-1-pclouds@gmail.com>

Signature cleanups

When extracting a commit's signature, we first free the object and only
afterwards put its signature contents into the result buffer. This works
in most cases - the free'd object will normally be cached anyway, so we
only end up decrementing its reference count without actually freeing
its contents. But in some more exotic setups, where caching is disabled,
this can definitly be a problem, as we might be the only instance
currently holding a reference to this object.

Fix this issue by first extracting the contents and freeing the object
afterwards only.

The functions `git_commit_header_field` and
`git_commit_extract_signature` both receive buffers used to hand back
the results to the user. While these functions called `git_buf_sanitize`
on these buffers, this is not the right thing to do, as it will simply
initialize or zero-terminate passed buffers. As we want to overwrite
contents, we instead have to call `git_buf_clear` to completely reset
them.

Changes to provide option to turn off/on ofs_delta

When `git_buf_sanitize` gets called, it converts a buffer with NULL
content to be correctly initialized. This is done by pointing it to
`git_buf__initbuf`. While the method's documentation states this
clearly, it may also lead to the conclusion that it will do the same to
buffers which do _not_ have NULL contents.

Clarify behavior when passing a buffer with non-NULL contents, where
`git_buf_sanitize` will ensure that the contents are `\0`-terminated.

Worktree implementation

tests: fix permissions on testrepo.git index file

When opening a worktree via the gitdir of its parent repository
we fail to correctly set up the worktree's working directory. The
problem here is two-fold: we first fail to see that the gitdir
actually is a gitdir of a working tree and then subsequently
fail to determine the working tree location from the gitdir.

The first problem of not noticing a gitdir belongs to a worktree
can be solved by checking for the existence of a `gitdir` file in
the gitdir. This file points back to the gitlink file located in
the working tree's working directory. As this file only exists
for worktrees, it should be sufficient indication of the gitdir
belonging to a worktree.

The second problem, that is determining the location of the
worktree's working directory, can then be solved by reading the
`gitdir` file in the working directory's gitdir. When we now
resolve relative paths and strip the final `.git` component, we
have the actual worktree's working directory location.

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.

Restrict the ability to delete branches that are checked out in
any linked repository.

Implement a new function that is able to determine if a branch is
checked out in any repository connected to the current
repository. In particular, this is required to check if for a
given repository and branch, there exists any working tree
connected to that repository that is referencing this branch.

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.

Implement the `git_worktree_add` function which can be used to create
new working trees for a given repository.

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.

Add function `git_repository_open_from_worktree`, which allows to open a
`git_worktree` as repository.

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.