This moves the implementation of these two APIs into common code
that will be shared between the two.  Also, this adds tests for
the `git_diff_blob_to_buffer` API.  Lastly, this adds some extra
`const` to a few places that can use it.

Revert changes from git/git diff-delta.c by dak@gnu.org, proski@gnu.org

Add Florian Forster to hall of fame

Permission recieved from Florian Forster on Jan 07 2013 to include
any changes of his from core Git into LibGit2.

merge cleanup should actually cleanup and the test should actually test

This reverts commit 47fbcbb5.

travis: Only run Valgrind when all tests pass

Allow to clone repositories that don't have a `master` branch

travis: Include the online suite when running against Travis

Before this, we error out from `reference_matches_remote_head` if the
reference we're searching for does not exist.
Since we explicitly check if master is existing in `update_head_to_remote`
and error out if it doesn't, a repository without master branch could
not be cloned.
In fact this was later clobbered by what is fixed in #1194.

However, this patch introduces a `found` member in `head_info` and sets
it accordingly. That also saves us from checking the string length of
`branchname` a few times.

git_mwindow_file_deregister() shouldn't return errors

As a function that appears to only be called on error paths, I don't
think it makes sense for it to return an error, or clobber the global
giterr. Note that no existing callsites actually check the return
code.

In my own application, there are errors where the real error ends
up being hidden, as git_mwindow_file_deregister() clobbers the
global giterr. I'm not sure this error is even relevant?

Fix an issue with ahead-behind for lopsided traversal

Handle packed refs with no trailing newline

I saw a repo in the wild today which had a master branch ref which was packed, but had no trailing newline. Git handled it fine, but libgit2 choked on it. Fix seems simple enough. If we don't see a newline, assume the end of the buffer is the end of the ref line.

Fix win32 checkout test

It turns out that using REMOVE_UNTRACKED with checkout for this
particular test was causing the .gitattributes file to be removed
and so we do have to allow for the CRs in the created file...

Move some checkout utility functions into a shared file and fix
some crlf filtering issues when verifying file contents.

Fixed size_t format string warning

Fix checkout corner cases, index handling, and reset hard behavior

There are a couple of checkout bugs fixed here.  One is with
untracked working directory entries that are prefixes of tree
entries but not in a meaningful way (i.e. "read" is a prefix of
"readme.txt" but doesn't interfere in any way).  The second bug
is actually a redo of 07edfa0fc640f85f95507c3101e77accd7d2bf0d
where directory entries in the index that are not in the diff
were not being removed correctly.  That fix remedied one case
but broke another.

This adds the git_index_remove_directory API plus tests.

I made a small change to the behavior of this code and apparently
got it wrong. Sigh.

When checking out with the GIT_CHECKOUT_REMOVE_UNTRACKED option
and there was an entire tree in the working directory and in the
index that is not in the baseline nor target commit, the tree was
correctly(?) removed from the working directory but was not
successfully removed from the index.  This fixes that and adds a
test of the functionality.

This adds a bunch of new checkout tests and in the process I found
a bug in the GIT_CHECKOUT_UPDATE_ONLY flag which I fixed.

This moves a lot of the detailed checkout documentation into a new
file (docs/checkout-internals.md) and simplifies the public docs
for the checkout API.

For clone to work as expected, it should be using a SAFE_CREATE
checkout (i.e. create files that are missing, even if the target
tree matches the current HEAD).