If trying to set the HEAD of a repository to another reference, we have
to check whether this reference is already checked out in another linked
work tree. If it is, we will refuse setting the HEAD and return an
error, but do not set a meaningful error message. Add one.

Currently, we only provide functions to read references directly from a
repository's reference store via e.g. `git_reference_lookup`. But in
some cases, we may want to read files not connected to the current
repository, e.g. when looking up HEAD of connected work trees. This
commit implements `git_reference__read_head`, which will read out and
allocate a reference at an arbitrary path.

git_treebuilder_write_with_buffer refactorings

While writing the tree inside of a buffer, we check whether the buffer
runs out of memory after each tree entry. While we set the error code as
soon as we detect the OOM situation, we happily proceed iterating over
the entries. This is not useful at all, as we will try to write into the
buffer repeatedly, which cannot work.

Fix this by exiting as soon as we are OOM.

The `git_tree_entry *entry` variable is defined twice inside of this
function. While this is not a problem currently, remove the shadowing
variable to avoid future confusion.

While we detect errors in `git_treebuilder_write_with_buffer`, we just
exit directly instead of freeing allocated memory. Fix this by
remembering error codes and skipping forward to the function's cleanup
code.

Fix memory leaks

The recent addition of an error code to `pass_whole_blame` in ff8d2eb1
(blame_git: check return value of object lookup, 2017-03-20) introduced
a spurious goto. Remove it.

git_futils: don't O_EXCL and O_TRUNC

sha1dc: perf improvements from upstream

inet_pton: don't assume addr families don't exist

Update SHA-1 collision detection code (cr-marcstevens/sha1collisiondetection)
to master to include performance improvements.

Worktree fixes

fsync all the things

Coverity fixes

Fix the documentation for git_cred_acquire_cb

Address family 5 might exist on some crazy system like Haiku.
Use `INT_MAX-1` as an unsupported address family.

`O_EXCL` and `O_TRUNC` are mutually exclusive flags to open(2); you can't
truncate a file if you're asserting that it can't exist in the first place.
Drop `O_TRUNC`.

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.