netops: return GIT_ECERTIFICATE when it fails the basic tests

Fix segmentation fault observed on OpenBSD/sparc64

Make the Visual Studio compiler happy

A non-readable mapping of a file causes an access violation in
the pack tests. Always use PROT_READ to work around this.

An anonymous remote wouldn't create remote-tracking branches, so testing
we don't create them for TAGS_ALL is nonsensical. Furthermore, the name
of the supposed remote-tracking branch was also not one which would have
been created had it had a name.

Give the remote a name and test that we only create the tags when we
pass TAGS_ALL and that we do create the remote-branch branch when given
TAGS_AUTO.

Since it's not necessarily obvious, mention the merging state and how to
clear it.

When we first ask OpenSSL to verify the certfiicate itself (rather
than the HTTPS specifics), we should also return
GIT_ECERTIFICATE. Otherwise, the caller would consider this as a failed
operation rather than a failed validation and not call the user's own
validation.

revert tests: always use core.autocrlf=false in tests

Fixed the most obvious Clang static analyzer warnings

Flip THREADSAFE's default to ON

Threads are here to stay; and for a while now, users have had to call
the initialization function which sets up threads and crypto regardless
of whether the library was built threadsafe or not.

Signed-off-by: Sven Strickroth <email@cs-ware.de>

Rebase (merge)

Rename git_merge_head to git_annotated_commit, as it becomes used
in more operations than just merge.

Already cherry-picked commits should not be re-included.  If all changes
included in a commit exist in the upstream, then we should error with
GIT_EAPPLIED.

Commit the current patch of a rebase process.

`git_rebase_next` will apply the next patch (or cherry-pick)
operation, leaving the results checked out in the index / working
directory so that consumers can resolve any conflicts, as appropriate.

Abort an in-progress rebase and move the working directory and
repository back to the ORIG_HEAD state.

Introduce `git_rebase` to set up a rebase session that can
then be continued.  Immediately, only merge-type rebase is
supported.