Finish to hide git_pkt from external API.

reflog: Fix reflog writer/reader

Some of the WIP API calls have been hidden in preparation for the next
minor release.

 - Use a space to separate oids and signature
 - Enforce test coverage
 - Make test run in a temporary folder in order not to alter the test repository

Fix MSVC compilation warning

The `hashfile` function has been moved to ODB, next to `git_odb_hash`.

Global state has been removed from the dirent call in `status.c`,
because global state is killing the rainforest and causing global
warming.

Throws GIT_ENOTFOUND error if given a filename that is not in
HEAD, index, nor the work tree.

Refactored copy of test repo to a function.

Add git_status_file to be able to retrieve status of single file by
supplying a path.

Add git_status_foreach() to run a callback on each file passing the path
and a status value.

Add git_status_hashfile() to get blob's object id for a file without adding
it to the object database or needing a repository at all.
This functionality is similar to `git hash-object` without '-w'.

Ntk/fix win32 specs

The direct-writes commit left some (slow) internals methods that
were no longer needed. These have been removed.

Also, the Reflog code was using the old `git_signature__write`, so
it has been rewritten to use a normal buffer and the new `writebuf`
signature writer. It's now slightly simpler and faster.

DIRECT WRITES ARE BACK AND FASTER THAN EVER. The streaming writer to the
ODB was an overkill for the smaller objects like Commit and Tags; most
of the streaming logic was taking too long.

This commit makes Commits, Tags and Trees to be built-up in memory, and
then written to disk in 2 pushes (header + data), instead of streaming
everything.

This is *always* faster, even for big files (since the git_filebuf class
still does streaming writes when the memory cache overflows). This is
also a gazillion lines of code smaller, because we don't have to
precompute the final size of the object before starting the stream (this
was kind of defeating the point of streaming, anyway).

Blobs are still written with full streaming instead of loading them in
memory, since this is still the fastest way.

A new `git_buf` class has been added. It's missing some features, but
it'll get there.

Our good, lovely folks at Microsoft decided that there was no good
reason to make `vsnprintf` compilant with the C standard, so that
function in Windows returns -1 on overflow, instead of returning the
actual byte count needed to write the full string.

We now handle this situation more gracefully with the POSIX
compatibility layer, by returning the needed byte size using an
auxiliary method instead of blindly resizing the target buffer until it
fits.

This means we can now support `printf`s of any size by allocating a
temporary buffer. That's good.

So far libgit2 didn't support reference logs (reflog). Add a new
git_reflog_* API for basic reading and writing of reflogs:

* git_reflog_read
* git_reflog_write
* git_reflog_free

Signed-off-by: schu <schu-github@schulog.org>

Signed-off-by: schu <schu-github@schulog.org>

reference_rename: fix flaw in force-renaming

tsort: remove unused but set variable

reference_rename didn't respect the force flag. Fixed.

Reported-by: nulltoken <emeric.fermas@gmail.com>
Signed-off-by: schu <schu-github@schulog.org>

Add some more test checking forced reference renaming.

Signed-off-by: nulltoken <emeric.fermas@gmail.com>
Acked-by: schu <schu-github@schulog.org>

Signed-off-by: schu <schu-github@schulog.org>