Expose the function `repo_init_create_head` as
`git_repository_create_head`.

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 variable '.path' of the refdb_fs_backend struct becomes
confusing regarding the introduction of the git commondir. It
does not immediatly become obvious what it should point to.

Fix this problem by renaming the variable to `gitpath`,
clarifying that it acutally points to the `.git` directory of the
repository, in contrast to the commonpath directory, which points
to the directory containing shared objects like references and
the object store.

The recent introduction of the commondir variable of a repository
requires callers to distinguish whether their files are part of
the dot-git directory or the common directory shared between
multpile worktrees. In order to take the burden from callers and
unify knowledge on which files reside where, the
`git_repository_item_path` function has been introduced which
encapsulate this knowledge.

Modify most existing callers of `git_repository_path` to use
`git_repository_item_path` instead, thus making them implicitly
aware of the common directory.

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.

Add a new branch that causes a merge conflict to `testrepo` so
that we are able to test merging in worktrees.

Create worktrees for submodule repositories. The worktrees are
created for the parent repository (e.g. the one containing
submodules) and for the contained child repository.

Add support for lowercase proxy environment variables

dirname with DOS prefixes

Flag options in git_stash_apply and git_stash_pop as being optional

rename detection: don't try to detect submodule renames

Test that shows that submodules are incorrectly considered in renames,
and `git_merge_trees` will fail to lookup the submodule as a blob.

The options parameter in both git_stash_apply and git_stash_pop can
be NULL. They should be flagged as such in the documentation.

Fix digest credentials for proxy in windows

curl supports HTTPS_PROXY in addition to https_proxy (and their http counterparts). This change ensures parity with curl's behavior.

As of recently, we failed to correctly discover repositories at a
Win32 system root. Instead of aborting the upwards-traversal of
the file system, we were looping infinitely when traversal
started at either a Win32 drive prefix ("C:/") or a network path
("//somehost").

The issue has been fixed, so add a test to catch regressions.

When calling `git_path_dirname_r` on a Win32 prefix, e.g. a drive
or network share prefix, we always want to return the trailing
'/'. This does not work currently when passing in a path like
'C:', where the '/' would not be appended correctly.

Fix this by appending a '/' if we try to normalize a Win32 prefix
and there is no trailing '/'.

Getting the dirname of a filesystem root should return the filesystem
root itself. E.g. the dirname of "/" is always "/". On Windows, we
emulate this behavior and as such, we should return e.g. "C:/" if
calling dirname on "C:/". But we currently fail to do so and instead
return ".", as we do not check if we actually have a Windows prefix
before stripping off the last directory component.

Fix this by calling out to `win32_prefix_length` immediately after
stripping trailing slashes, returning early if we have a prefix.

Extract code which determines if a path is at a Windows system's root.
This incluses drive prefixes (e.g. "C:\") as well as network computer
names (e.g. "//computername/").

Flag checkout_opts in git_reset as optional

The check_outs argument can be NULL and should be flagged accordingly
in the header file.

Flag given_opts in git_revert as optional

The given_opts argument can actually be NULL and thus should be
flagged accordingly in the header file.

Vector reverse overflow

The code reversing a vector initially determines the rear-pointer by
simply subtracting 1 from the vector's length. Obviously, this fails if
the vector is empty, in which case we have an integer overflow.

Fix the issue by returning early if the vector is empty.

Update docs for git_oid_fromstrn and p