signature: don't allow empty emails

A signature is made up of a non-empty name and a non-empty email so
let's validate that. This also brings us more in line with git, which
also rejects ident with an empty email.

Restrict which refs can be the default branch

ssh: store error message immediately after a failed agent call

When the call to the agent fails, we must retrieve the error message
just after the function call, as other calls may overwrite it.

As the agent authentication is the only one which has a teardown and
there does not seem to be a way to get the error message from a stored
error number, this tries to introduce some small changes to store the
error from the agent.

Clearing the error at the beginning of the loop lets us know whether the
agent has already set the libgit2 error message and we should skip it,
or if we should set it.

Clean up transport lookup

Teach repository to use relative paths for git symbolic links

Several CppCat warnings fixed

Teach git_repository_init_ext to use relative paths for the gitlink
to the work directory. This is used when creating a sub repository
where the sub repository resides in the parent repository's
.git directory.

.editorconfig added to repository

Skip SSH clone tests if libgit2 not built with GIT_SSH defined

When the fetch refspec does not include the remote's default branch, it
indicates an error in user expectations or programmer error. Error out
in that case.

This lets us get rid of the dummy refspec which can never work as its
zeroed out. In the cases where we did not find a default branch, we set
HEAD detached immediately, which lets us refactor the "normal" path,
removing `found_branch`.

If the remote does not advertise HEAD, then it is unborn and we cannot
checkout that branch. Handle it the same way as an empty repo.

When cloning, we may be asking for a particular branch or subset of
branches. Make sure we test for that.

Add tests for the case when there are no branches on the remote and when
HEAD is detached but has the id of a non-branch. In both of these cases,
we should return ENOTFOUND.

Fix warning

Instead of using ifdefs to run the tests, use them to set when we expect
to support a particular scheme and always have the tests in the code.

It does the same as git_remote_supported_url() but has a name which
implies we'd check the URL for correctness while we're simply looking at
the scheme and looking it up in our lists.

While here, fix up the tests so we check all the combination of what's
supported.

The previous commit makes it harder to figure out if the library was
built with support for a particular transport. Roll back some of the
changes and remove ssh:// and https:// from the list if we're being
built without support for them.

Even when built without a SSH support, we know about this transport. It
is implemented, but the current code makes us return an error message
saying it's not.

This is a leftover from the initial implementation of the transports
when there were in fact transports we knew about but were not
implemented.

Instead, let the SSH transport itself say it cannot run, the same as we
do for HTTPS.

MSVC does not support zero size array

merge: expose multiple merge bases

A repository can have any number of references which we're not
interested in such as notes or tags. For the default branch calculation
we only care about branches. Make the decision about the number of
branches rather than the number of refs in general.

If we do not have a HEAD ref in the heads, we already know there is no
default branch. Return immedately.

Assert what we already do, so as to notice changes.

This was added to avoid the remote's default branch to be considered to
the be notes one which the first network test leaves behind.

Fix ahead-behind results

When we see PARENT1, it means there is a local commit and thus we are
ahead. Likewise, seeing PARENT2 means that the upstream branch has a
commit and we are one more behind.

The logic is currently reversed. Correct it.

This fixes #2501.

The logic was reversed. I have checked manually each pair with git and
adjusted the expectation to what git status prints.

winhttp: Prevent swallowing of url parsing error

Set timeout on remote (Add timeout for WinHttpReceiveResponse #2147)