Rename git_threads_ to git_libgit2_

This describes their purpose better, as we now initialize ssl and some
other global stuff in there. Calling the init function is not something
which has been optional for a while now.

Fixed GIT_REMOTE_DOWNLOAD_TAGS_ALL to behave like git 1.9.0+

Refactor fetchhead

remote: rename _load() to _lookup()

odb: hardcode the empty blob and tree

git_status_file now takes an exact path.

This is an ugly chunk of code, so let's put it into its own function.

git hardocodes these as objects which exist regardless of whether they
are in the odb and uses them in the shell interface as a way of
expressing the lack of a blob or tree for one side of e.g. a diff.

In the library we use each language's natural way of declaring a lack of
value which makes a workaround like this unnecessary. Since git uses it,
it does however mean each shell application would need to perform this
check themselves.

This makes it common work across a range of applications and an issue
with compatibility with git, which fits right into what the library aims
to provide.

Thus we introduce the hard-coded empty blob and tree in the odb
frontend. These hard-coded objects are checked for before going to the
backends, but after the cache check, which means the second time they're
used, they will be treated as normal cached objects instead of creating
new ones.

If the remote is anonymous, then we cannot check for any configuration,
as there is no name. Check for this before we try to use the name, which
may be a NULL pointer.

This fixes #2697.

This reduces the clutter somewhat and lets us see what we're asking
about the reference.

This gets the value from branch.<foo>.remote.

This brings it in line with the rest of the lookup functions.

This function has one output but can match multiple files, which can be
unexpected for the user, which would usually path the exact path of the
file he wants the status of.

submodules: stale module entries

We cannot know from looking at .gitmodules whether a directory is a
submodule or not. We need the index or tree we are comparing against to
tell us. Otherwise we have to assume the entry in .gitmodules is stale
or otherwise invalid.

Thus we pass the index of the repository into the workdir iterator, even
if we do not want to compare against it. This follows what git does,
which even for `git diff <tree>`, it will consider staged submodules as
such.

We consider an entry in .gitmodules to mean that we have a submodule at
a particular path, even if HEAD^{tree} and the index do not contain any
reference to it.

We should ignore that submodule entry and simply consider that path to
be a regular directory.

checkout_index: handle other stages

ignore: don't leak rules into higher directores

Threading and crypto libraries

remote: check for the validity of the refspec when updating FETCH_HEAD

A rule "src" in src/.gitignore must only match subdirectories of
src/. The current code does not include this context in the match rule
and would thus consider this rule to match the top-level src/ directory
instead of the intended src/src/.

Keep track fo the context in which the rule was defined so we can
perform a prefix match.

When we mention "src" in src/.gitignore, we wrongly consider src/ itself
to be ignored.

ignore: consider files with a CR in their names

We currently consider CR to start the end of the line, but that means
that we miss cases with CR CR LF which can be used with git to match
files whose names have CR at the end of their names.

The fix from the patch comes from Russell's comment in the issue.

This fixes #2536.

Propogate GIT_ENOTFOUND from git_remote_rename

Make config reading continue after hitting a missing include file.

Before trying to rtransform using the given refspec to figure out what
the name of the upstream branch is on the remote, we must make sure that
the target of the refspec applies to the current branch's upstream.

Changed context_lines and interhunk_lines to uint32_t to match struct s_xdemitconf

remote: unify the creation code

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.

* Error-handling is cleaned up to only let a file-not-found error
  through, not other sorts of errors.  And when a file-not-found
  error happens, we clean up the error.
* Test now checks that file-not-found introduces no error.  And
  other minor cleanups.