Data-driven tests

We did not properly support default credentials for proxies, only for
destination servers.  Refactor the credential handling to support sending
either username/password _or_ default credentials to either the proxy or
the destination server.

This actually shares the authentication logic between proxy servers and
destination servers.  Due to copy/pasta drift over time, they had
diverged.  Now they share a common logic which is: first, use
credentials specified in the URL (if there were any), treating empty
username and password (ie, "http://:@foo.com/") as default credentials,
for compatibility with git.  Next, call the credential callbacks.
Finally, fallback to WinHTTP compatibility layers using built-in
authentication like we always have.

Allowing default credentials for proxies requires moving the security
level downgrade into the credential setting routines themselves.
We will update our security level to "high" by default which means that
we will never send default credentials without prompting.  (A lower
setting, like the WinHTTP default of "medium" would allow WinHTTP to
handle credentials for us, despite what a user may have requested with
their structures.)  Now we start with "high" and downgrade to "low" only
after a user has explicitly requested default credentials.

There's no reason a git repository couldn't be at the root of a server,
and URLs should have an implicit path of '/' when one is not specified.

"Connection data" is an imprecise and largely incorrect name; these
structures are actually parsed URLs.  Provide a parser that takes a URL
string and produces a URL structure (if it is valid).

Separate the HTTP redirect handling logic from URL parsing, keeping a
`gitno_connection_data_handle_redirect` whose only job is redirect
handling logic and does not parse URLs itself.

The function `populate_symlink_workdir` creates a new
"symlink.git" repository with a relative path "../symlink.git".
As the current working directory is the sandbox, the new
repository will be created just outside of the sandbox.

Fix this by using `clar_sandbox_path`.

The object::cache test module has two tests that do nearly the
same thing: given a cache limit, load a certain set of objects
and verify if those objects have been cached or not.

Convert those tests to the new data-driven initializers to
demonstrate how these are to be used. Furthermore, add some
additional test data. This conversion is mainly done to show this
new facility.

Right now, we're not able to use data-driven tests at all. E.g.
given a set of tests which we'd like to repeat with different
test data, one has to hand-write any outer loop that iterates
over the test data and then call each of the test functions. Next
to being bothersome, this also has the downside that error
reporting is quite lacking, as one never knows which test data
actually produced failures.

So let's implement the ability to re-run complete test modules
with changing test data. To retain backwards compatibility and
enable trivial addition of new runs with changed test data, we
simply extend clar's generate.py. Instead of scanning for a
single `_initialize` function, each test module may now implement
multiple `_initialize_foo` functions. The generated test harness
will then run all test functions for each of the provided
initializer functions, making it possible to provide different
test data inside of each of the initializer functions. Example:

```
void test_git_example__initialize_with_nonbare_repo(void)
{
	g_repo = cl_git_sandbox_init("testrepo");
}

void test_git_example__initialize_with_bare_repo(void)
{
	g_repo = cl_git_sandbox_init("testrepo.git");
}

void test_git_example__cleanup(void)
{
	cl_git_sandbox_cleanup();
}

void test_git_example__test1(void)
{
	test1();
}

void test_git_example__test2(void)
{
	test2();
}
```

Executing this test module will cause the following output:

```
$ ./libgit2_clar -sgit::example
git::example (with nonbare repo)..
git::example (with bare repo)..
```

ignore: handle escaped trailing whitespace

Ignore: only treat one leading slash as a root identifier