refs: explicitly catch leading slashes

It's somewhat common to try to write "/refs/tags/something". There is
no easy way to catch it during the main body of the function, as there
is no way to distinguish whether it's a leading slash or a double
slash somewhere in the middle.

Catch this at the beginning so we don't trigger the assert in
is_all_caps_and_underscore().

[RFC] basic note iterator implementation

* fixed style issues
* use new iterator functions for git_note_foreach()

* git_note_iterator_new() - create a new note iterator
* git_note_next() - retrieves the next item of the iterator

remote push test fix

More tests (and fixes) for initializing repo from template

Disable ignore_case when writing the index to a tree

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