Basic setup for profiling

This fixes the examples so they will build and adds a PROFILE
option to the CMakeFile that enabled gprof info on non-Windows
Optimize away git_text_gather_stats in diff

GProf shows `git_text_gather_stats` as the most expensive call
in large diffs.  The function calculates a lot of information
that is not actually used and does not do so in a optimal
order.  This introduces a tuned `git_buf_is_binary` function
that executes the same algorithm in a fraction of the time.
Other optimization and warning fixes

This fixes a warning left by the earlier optimization and
addresses one of the other hotspots identified by GProf.
Fix workdir iterators on empty directories

Creating a workdir iterator on a directory with absolutely
no files was returning an error (GIT_ENOTFOUND) instead of
an iterator for nothing.  This fixes that and includes two
new tests that cover that case.
Merge pull request #708 from arrbee/fix-empty-workdir-iterator

Fix workdir iterators on empty directories
No point in keeping commented out fn
Merge pull request #709 from arrbee/profiling-with-scottg

Profiling with scottg
branch: retrieve symbolic references when listing the branches
global: Change parameter ordering in API

Consistency is good.
global: Fix unit tests after reordering
errors: Remove old comments
refs: git_reference_listall -> git_reference_list
Properly tag all `enums` with a `_t`
errors: Rename the generic return codes
errors: Rename error codes
Merge pull request #685 from nulltoken/fix/list-remote-branches

branch: retrieve symbolic references when listing the branches
fetch: set dummy function for local fetch

Local fetch isn't implemented yet. Don't segfault on call, but set a
dummy for negotiate_fetch and terminate gracefully.

Reported-by: Brad Harder <bch@methodlogic.net>
Merge pull request #711 from schu/fetch-local-dummy

fetch: set dummy function for local fetch
Merge pull request #710 from libgit2/breaking-changes

Break everything before the release
libgit2 v0.17.0 "Lord of Diffstruction"

Welcome to yet another libgit2 release, this one being the
biggest we've shipped so far. Highlights on this release
include diff, branches, notes and submodules support. The new
diff API is shiny and powerful. Check it out.

Apologies, one more time, to all the early adopters for the
breaking API changes. We've been iterating on the error
handling for the library until we reached its current state,
which we believe it's significantly more usable both for normal
users and for developers of bindings to other languages.
Also, we've renamed a few legacy calls to ensure that the whole
external API uses a consistent naming scheme.

As always, check the API docs for the full list of new API calls
and backwards-incompatible changes.

	http://libgit2.github.com/libgit2/

Changelog of new features follows:

Attributes:
	- Added function macros to check attribute values instead of having
	to manually compare them
	- Added support for choosing the attribute loading order (workdir files
	vs index) and to skip the systems' default `.gitattributes`
	- Fixed issues when fetching attribute data on bare repositories

Blob:
	- Added support for creating blobs from any file on disk (not
	restricted to the repository's working directory)
	- Aded support for smudge filters when writing blobs to the ODB
		- So far only CRLF normalization is available

Branches:
	- Added a high-level branch API:
		- git_branch_create
		- git_branch_delete
		- git_branch_list
		- git_branch_move

Commit:
	- Commit messages are now filtered to match Git rules (stripping
	comments and adding proper whitespacing rules)

Config:
	- Added support for setting and getting multivars
	- Added `git_config_get_mapped` to map the value of a config
	variable based on its defaults

Diff:
	- Added full diff API:
		- tree to tree
		- index to tree
		- workdir to index
		- workdir to tree
		- blob to blob

	- Added helper functions to print the diffs as valid patchfiles

Error handling:
	- New design for the error handling API, taking into consideration
	the requirements of dynamic languages

Indexer:
	- Added streaming packfile indexer

Merge:
	- Added support for finding the merge base between two commits

Notes:
	- Full git-notes support:
		- git_note_read
		- git_note_message/git_note_oid
		- git_note_create
		- git_note_remove
		- git_note_free
		- git_note_foreach

References:
	- Added `git_reference_name_to_oid` helper to resolve
	a reference to its final OID

	- Added `git_reference_cmp` to compare two references with
	a stable order

Remotes:
	- Added support for writing and saving remotes
		- `git_remote_add`
		- `git_remote_save`
		- Setters for all the attributes of a remote
	- Switched remote download to the new streaming packfile indexer
	- Fixed fetch on HTTP and Git under Windows
	- Added `git_remote_supported_url` helper to check if a protocol
	can be accessed by the library
	- Added `git_remote_list`

Repository:
	- Made `git_repository_open` smarter when finding the `.git` folder.
	- Added `git_repository_open_ext` with extra options when
	opening a repository

Revwalk:
	- Added support for pushing/hiding several references through a glob
	- Added helper to push/hide the current HEAD to the walker
	- Added helper to push/hide a single reference to the walker

Status:
	- Greatly improved Status implementation using the new `diff` code
	as a backend

Submodules:
	- Added a partial submodules API to get information about a
	submodule and list all the submodules in a repository
		- git_submodule_foreach
		- git_submodule_lookup

Tag:
	- Added `git_tag_peel` helper to peel a tag to its pointed object
	- Tag messages are now filtered to match Git rules (stripping comments
	and adding proper whitespacing rules)

Tree:
	- Killed the old `git_tree_diff` API, which is replaced by the
	new diff code.

Signed-off-by: Vicent Marti <tanoku@gmail.com>