alloc doesn't take a refdb

This fixes #1487

Add filesystem iterator variant

When attempting to create a workdir iterator for a bare repo,
don't leak the iterator structure.

Refactors the helper function that builds a directory hierarchy
and then made use of it to try more variations on filesystem
iterator tests.

Renamed the callback functions and made some minor rearrangements
to clean up the flow of some code.

This adds some hooks into the filesystem iterator so that the
workdir iterator can just become a wrapper around it.  Then we
remove most of the workdir iterator code and just have it augment
the filesystem iterator with skipping .git entries, updating the
ignore stack, and checking for submodules.

This adds a new variant iterator that is a raw filesystem iterator
for scanning directories from a root.  There is still more work to
do to blend this with the working directory iterator.

Return error for empty name/email

use a longer string for dummy data in test to avoid conflicting w/ index

Allow access to the cached peel data in packed-refs

checkout: use cache when possible to determine if workdir item is dirty

Add `git_repository_open_bare`

If the on-disk file has been staged (it's stat data matches the stat data
in the cache) then we need not hash the file to determine whether it
differs from the checkout target; instead we can simply use the oid in
the index.

This prevents recomputing a file's hash unnecessarily, prevents loading
the file (when filtering) and prevents edge cases where filters suggest
that a file is dirty immediately after git writes the file.

Update link to Perl bindings

This is the last minor release before 1.0preview1.

Highlights of this release include:

- Branch API
- Checkout head, index and tree
- Finished clone support
- Abstracted reference API to use custom backends
- Full diff support
- New (faster) packbuilder
- Push support
- New Remotes API
- Revparse support (single and range commits)
- Stash support
- Submodules support

As always, the full changelog is available at:

	http://libgit2.github.com/libgit2/#p/changelog

Yeah, it's a huge release. Releasing stuff sucks.
Expect 1.0 and API freeze in less than a month.

Your faithful maintainer,
	 vmg

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

Unified rev-parse, with a revision object

Fetch fixes

Don't try to update anything if there are no heads to update. This
saves us from trying to look into a fetch refspec when there is none.

A better fix for compatibility with git when using remotes without
refspecs is still needed, but this stops us from segfaulting.