Make git_object_peel a bit smarter

refs: expose git_reference_normalize_name()

revwalk: refuse push of non-commit objects

This expands the types of peeling that `git_object_peel` knows
how to do to include TAG -> BLOB peeling, and makes the errors
slightly more consistent depending on the situation.  It also
adds a new special behavior where peeling to ANY will peel until
the object type changes (e.g. chases TAGs to a non-TAG).

Using this expanded peeling, this replaces peeling code that was
embedded in `git_tag_peel` and `git_reset`.

branch: reduce code duplication

Check the type of the pushed object immediately instead of starting the
walk and failing in between.

It's not really needed with the current code as we have EOS and the
sideband's flush to tell us we're done.

Keep the distinction between processed and received objects.

This poor transport was forgotten in the recent sideband support.

Make sure that idx->pack isn't NULL before trying to free resources
under it.

Revparse: GIT_EAMBIGUOUS

Revparse now returns EAMBIGUOUS if the the spec
doesn't match any refs/tags, and is <4 characters.

Add sideband support

Submodule extensions

new config file locations and defaults

As of git v1.7.12, $HOME/.config/git/ is supported as a new
location for "config", "attributes", and "ignore" files.

Depending on what you had done before adding new items to the
internal ignores list, it was possible for the cache of ignore
data to be uninitialized.

Just clean up valgrind warnings about uninitialized memory
and also clear out errno in some cases where it results in
a false error message being generated at a later point.

This lets us notify the user of what the remote end is doing while we
wait for it to start sending us the packfile.

This is a big redesign of the git_submodule_status API and the
implementation of the redesigned API.  It also fixes a number of
bugs that I found in other parts of the submodule API while
writing the tests for the status part.

This also fixes a couple of bugs in the iterators that had not
been noticed before - one with iterating when there is a gitlink
(i.e. separate-work-dir) and one where I was treating anything
even vaguely submodule-like as a submodule, more aggressively
than core git does.

This fixes up a number of problems flagged by valgrind and also
cleans up the internal `git_submodule` allocation handling
overall with a simpler model.

This replaces the old submodule API with a new extended API that
supports most of the things that can be done with `git submodule`.

We can't always rely on the network telling us when the download is
finished. Recognize it from the indexer itself.

Add git_repository_init_ext for power initters

This cleans up a number of items suggested during code review
with @vmg, including:

* renaming "outside repo" config API to `git_config_open_default`
* killing the `git_config_open_global` API
* removing the `git_` prefix from the static functions in fileops
* removing some unnecessary functionality from the "cp" command

If you use the clar cleanup callback function, you can't pass a
reference pointer to a stack allocated variable because when the
cleanup function runs, the stack won't exist anymore.

This extends git_repository_init_ext further with support for
initializing the repository from an external template directory
and with support for the "create shared" type flags that make a
set GID repository directory.

This also adds tests for much of the new functionality to the
existing `repo/init.c` test suite.

Also, this adds a bunch of new utility functions including a
very general purpose `git_futils_mkdir` (with the ability to
make paths and to chmod the paths post-creation) and a file
tree copying function `git_futils_cp_r`.  Also, this includes
some new path functions that were useful to keep the code
simple.

The extended version of repository init adds support for many
of the things that you can do with `git init` and sets up
structures that will make it easier to extend further in the
future.

In looking at PR #878, I found a few small bugs in the diff code,
mostly related to work that can be avoided when processing tree-
to-tree diffs that was always being carried out.  This commit has
some small fixes in it.

API for managing in-memory ignore rules

This fills out the ignore API and adds tests.

This creates a public API for adding to the internal ignores
list, which already existing but was not accessible.

This adds the new default value for core.excludesfile also.

Filemode enum