clear REUC on checkout

`git_diff_get_patch()` would unconditionally load the patch object and
then simply leak it if the user hadn't requested it. Short-circuit
loading the object if the user doesn't want it.

The rest of the plugs are simply calling the free functions of objects
allocated during the tests.

indexer: use a hashtable for keeping track of offsets

These offsets are needed for REF_DELTA objects, which encode which
object they use as a base, but not where it lies in the packfile, so
we need a list.

These objects are mostly from older packfiles, before OFS_DELTA was
widely spread. The time spent in indexing these packfiles is greatly
reduced, though remains above what git is able to do.

indexer: kill git_indexer

This was the first implementation and its goal was simply to have
something that worked. It is slow and now it's just taking up
space. Remove it and switch the one known usage to use the streaming
indexer.

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