Fixups for rebase

Switch to a standard branch-switching pattern of a `SAFE` checkout,
then updating `HEAD`.

Don't require the branch to rebase, if given `NULL`, simply look up
`HEAD`.

Users may want to try to pay attention to the `exec` field on all
rebase operations.

win32: limit the mode to `_wopen`/`_waccess`

Win32 generally ignores Unix-like mode bits that don't make any
sense on the platform (eg `0644` makes no sense to Windows).  But
WINE complains loudly when presented with POSIXy bits.  Remove them.

(Thanks @phkelley)

Reinit `reader` pointer after reading included config file

Fix build warning on Android

Always do a time_t cast on st_mtime. st_mtime on Android is not
the type of time_t but has the same meaning which is the number
of seconds past epoch.

Check rebase options version on public entry points

Fixes #2869. If included file includes more files, it may reallocate
cfg_file->readers, hence invalidate not only `r` pointer, but `result`
pointer as well.

It fails at least on my computer, though it may depend on some unpredictable
factors (say, will realloc() extend the memory segment in place, or it will
allocate new memory).

Checkout performance

Remove stale reflog namespace directory before branch creation

Ensure we can make a repo at the root of the filesystem

Add libssh2's library and include directories.

Large merge perf test.

Update some third-party copyrights

Fix doc comment formatting

Speed up AppVeyor builds.

AppVeyor build machines come with Python 2.7 and CMake 2.8 pre-installed and in the PATH.

repository: parse init.templatedir as a path

This is a path so we must use the path getter so we get the tilde
expansion done.

Update clar to d17c7f6

Don't try to strip trailing paths from the root directory on
Windows (trying to create `C:` will fail).

`p_stat` calls `git_win32_path_from_utf8`, which canonicalizes the
path.  Do not further try to modify the path, else we trim the
trailing slash from a root directory and try to access `C:` instead
of `C:/`.

Test to ensure that we can create a repository at the filesystem
root.  Introduces a new test environment variable,
`GITTEST_INVASIVE_FILESYSTEM` for tests that do terrible things like
escaping the clar sandbox and writing to the root directory.  It is
expected that the CI builds will enable this but that normal people
would not want this.

Handle 8dot3 filenames being disabled on Win32

for-each-ref example: initialize libgit2 in main

I don't get how it was working without git_libgit2_init() call. I run it
and libgit2 throws assertion somewhere in its internals. Now it works.

Updated commit with shutdown at the end.

Fixes for pedantry