Update contributing and conventions

When the examples/diff.c was written, there was not yet a revparse
API.  Now we can use it to make command line parsing way better
with less code.  Yay!

The discussion about converting some of our foreach-style APIs to
use iterator objects got me wanting to make a list of good starter
projects.  I put it in CONTRIBUTING.md and then went crazy with
updates to that file and to CONVENTIONS.md.

Allow empty config object and use it for tests

This removes assertions that prevent us from having an empty
git_config object and then updates some tests that were
dependent on global config state to use an empty config before
running anything.

Control for core.autocrlf during testing

Clone should not clean up directories it did not create

Why cdecl why?

This fixes some snprintf and vsnprintf related deprecation
warnings we've been having on Windows with recent compilers.

This removes the one-off GIT_CDECL and adds a new standard way of
doing this named GIT_STDLIB_CALL with a src/win32 specific def
when on the Windows platform.

Allows compilation in newer versions of MinGW that already defined it.

Add file similarity scoring to diff rename/copy detection

don't dereference at the end of the workdir iterator

Win32: Use constants in version resource definitions where possible

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

Increment reference counter in git_repository_set_config

This fixes #1365

Give props to Martin Pool

Martin Pool was the original author of the code referenced in the clone
example. Make note that he's given his permission and also give him the
proper credit.

This reverts commit efe7fad6, except for
the indentation fixes.

hash: remove git_hash_init from internal api

Along with that, fix indentation in tests-clar/object/raw/hash.c

Initialize the hash ctx with git_hash_ctx_init, not git_hash_init.

packbuilder: minor improvements

Remove sample hook files

Getting rid of sample hook files from test repos as they just take up
space with no value.

Portability fixes for Solaris

Do not fail if .gitignore is directory

This is designed to fix libgit2sharp #350 where if .gitignore is
a directory we abort all operations that process ignores instead
of just skipping it as core git does.

Also added test that fails without this change and passes with it.

This includes tests for crlf changes, whitespace changes with the
default comparison and with the ignore whitespace comparison, and
more sensitivity checking for the comparison code.

This moves a couple of checks outside of the inner loop of the
find_similar rename/copy detection phase that are only dependent
on the "from" side of a detection.

Also, this replaces the inefficient initialization of the
options structure when a value is not provided explicitly by the
user.