Worktree fixes

fsync all the things

Coverity fixes

Fix the documentation for git_cred_acquire_cb

git_cred_acquire_cb isn't using the standard @param and @return tags.
This is causing the generated documentation to not be formatted
properly.

While parsing patch header lines, we iterate over each line and check if
the line has trailing garbage. What we do not check though is that the
line is actually a line ending with a trailing newline.

Fix this by checking the return code of `parse_advance_expected_str`.

The `pack_entry_find_prefix` function receives a `git_rawobj` structure
as argument. While the function first initializes the structure to a
sensible state, Coverity is unable to correctly detect this, resulting
in a warning.

Fix this warning by initializing the object to all-zeroes before passing
it to the function.

While parsing section headers, we use a buffer to store the actual
section name. We do not check though if the buffer runs out of memory at
any stage. Do so.

The function `pass_whole_blame` performs an object lookup but does not
check if the lookup actually succeeds. Convert the function to return an
error code and check for it in the calling function.

README: Mention how to run tests

Fix typo in remote.h API

The OpenSSL library may require multiple locks to work correctly, where
it is the caller's responsibility to initialize and release the locks.
While we correctly initialized up to `n` locks, as determined by
`CRYPTO_num_locks`, we were repeatedly freeing the same mutex in our
shutdown procedure.

Fix the issue by freeing locks at the correct index.

Remove `map_free` macros

The symlink destination is always concatenated to the original path. Fix
this by using `git_buf_sets` instead of `git_buf_puts`.

Diff fixes

merge_driver: fix const-correctness for source getters

The `map_free` functions were not implemented as functions but instead
as macros which also set the map to NULL. While this is most certainly
sensible in most cases, we should prefer the more obvious behavior,
namingly leaving the map pointer intact.

Furthermore, this macro has been refactored incorrectly during the
map-refactorings: the two statements are not actually grouped together
by a `do { ... } while (0)` block, as it is required for macros to
match the behavior of functions more closely. This has led to at least
one subtle nesting error in `pack-objects.c`. The following code block

```
    if (pb->object_ix)
        git_oidmap_free(pb->object_ix);
```

would be expanded to

```
    if (pb->object_ix)
        git_oidmap__free(pb->object_ix); pb->object_ix = NULL;
```

which is not what one woudl expect. While it is not a bug here as it
would simply become a no-op, the wrong implementation could lead to bugs
in other occasions.

Fix this by simply removing the macro altogether and replacing it with
real function calls. This leaves the burden of setting the pointer to
NULL afterwards to the caller, but this is actually expected and behaves
like other `free` functions.

We currently call `git_strmap_free` on `checkout_data.mkdir_map` in the
`checkout_data_clear` function. The only thing protecting us from a
double-free is that the `git_strmap_free` function is in fact not a
function, but a macro that also sets the map to NULL.

Remove the second call to `git_strmap_free` and explicitly set the map
member to NULL.

It is possible to specify submodule URLs relative to the repository
location. E.g. having a submodule with URL "../submodule" will look for
the submodule at "repo/../submodule".

With the introduction of worktrees, though, we cannot simply resolve the
URL relative to the repository location itself. If the repository for
which a URL is to be resolved is a working tree, we have to resolve the
URL relative to the parent's repository path. Otherwise, the URL would
change depending on where the working tree is located.

Fix this by special-casing when we have a working tree while getting the
URL base.

References for a repository are usually created inside of its gitdir.
When using worktrees, though, these references are not to be created
inside the worktree gitdir, but instead inside the gitdir of its parent
repository, which is the commondir. Like this, branches will still be
available after the worktree itself has been deleted.

The filesystem refdb currently still creates new references inside of
the gitdir. Fix this and have it create references in commondir.

The three link files "worktree/.git", ".git/worktrees/<name>/commondir"
and ".git/worktrees/<name>/gitdir" should always contain absolute and
resolved paths. Adjust the logic creating new worktrees to first use
`git_path_prettify_dir` before writing out these files, so that paths
are resolved first.

When creating a new worktree, we have to set up the initial data
structures. Next to others, this also includes the HEAD pseudo-ref.
We currently set it to the worktree respectively branch name, which is
actually not fully qualified.

Use the fully qualified branch name instead.

The working tree's parent path should not point to the parent's gitdir,
but to the parent's working directory. Pointing to the gitdir would not
make any sense, as the parent's working directory is actually equal to
both repository's common directory.

Fix the issue.

While we already provide functionality to look up a worktree from a
repository, we cannot do so the other way round. That is given a
repository, we want to look up its worktree if it actually exists.
Getting the worktree of a repository is useful when we want to get
certain meta information like the parent's location, getting the locked
status, etc.

Separate the logic of finding the worktree directory of a repository and
actually opening the working tree's directory. This is a preparatory
step for opening the worktree structure of a repository itself.

This will be used in later commits, where it becomes cumbersome to
always pass in a buffer.

submodule: catch when submodule is not staged on update

winhttp: disambiguate error messages when sending requests