Make `git_repository_head_tree()` return error codes

Create contributing guidelines

Reset all static variables to NULL in clar's __cleanup

Remove use of English expletives

Remove words such as fuck, crap, shit etc.
Remove other potentially offensive words from comments.
Tidy up other geopolicital terms in comments.

Without this change, any failed assertion in the second (or a later) test
inside a test suite has a chance of double deleting memory, resulting in
a heap corruption. See #1096 for details.

This leaves alone the test cases where we "just" use cl_git_sandbox_init()
and cl_git_sandbox_cleanup(). These methods already take good care to not
double delete a repository.

Fixes #1096

Update to Authors

It is okay to not have a .gitconfig file

Fix win32 lstat

Opening a repo is generating an error if you don't have a
.gitconfig file in your home directory, but that should be
legal.

VS-Build: Append C_FLAGS to the end of the already given ones.

My other PR revealed, that the /W4 parameter, we give to MSVC is ignored
because cmake set CMAKE_C_FLAGS already to /W3 and we overwrite it.

The command line tools gave me a D9025 warning for this on every file and
looking into the project properties page on MSVC 2008 tells, that it has
the warning level set to /W3.

However, the warnings introduced by /W4 are far to useless for having
them enabled. So just disable them.

Don't use precompiled headers for command-line based VC builds.

The reason, why libgit2 currently cannot support compiling via the command-line
tools cl/nmake from WinSDK and/or Microsoft Visual Studio, seems to be a
missing dependency on the generated precompiled header file.

The Visual Studio IDE automatically inserts this dependency when it sees the
right combination of "/Y" parameters.

This patch allows to compile using command line tools by disabling precompiled
headers for NON-IDE builds.

Filename validation

This prevents the index api from calling write_tree() with a
bogus tree.

As per core git.

Indexer: Avoid a possible double-deletion in error case

Itsy bitsy tiny winy minor api related fixes

Passing 0 as the index now retrieves the most recent entry instead
of the oldest one.