Fixed Xcode 6.1 build warnings

Fix memory leak in tests/network/refspecs.c

Zero out racily-clean entries' file_size

When checking out some file 'foo' that has been modified in the
working directory, allow the checkout to proceed (do not conflict)
if 'foo' is identical to the target of the checkout.

Provide functionality to set the time on a filesystem entry,
using utimes or futimes on POSIX type systems or SetFileTime
on Win32.

commit: ignore multiple author fields

remote: return EINVALIDSPEC when given an empty URL

This is what we used to return in the settter and there's tests in
bindings which ask for this. There's no particular reason to stop doing
so.

Fixed Xcode 6.1 build warnings

These tests want to test that we don't recalculate entries which match
the index already. This is however something we force when truncating
racily-clean entries.

Tick the index forward as we know that we don't perform the
modifications which the racily-clean code is trying to avoid.

In order to avoid racy-git, we zero out the file size for entries with
the same timestamp as the index (or during the initial checkout). This
is the case in a couple of crlf tests, as the code is fast enough to do
everything in the same second.

As we know that we do not perform the modification just after writing
out the index, which is what this is designed to work around, tick the
mtime of the index file such that it doesn't agree with the files
anymore, and we do not zero out these entries.

If a file entry has the same timestamp as the index itself, it is
considered racily-clean, as it may have been modified after the index
was written, but during the same second. We take extra steps to check
the contents, but this is just one part of avoiding races.

For files which do have changes but have not been updated in the index,
updating the on-disk index means updating its timestamp, which means we
would no longer recognise these entries as racy and we would trust the
timestamp to tell us whether they have changed.

In order to work around this, git zeroes out the file-size field in
entries with the same timestamp as the index in order to force the next
diff to check the contents. Do so in libgit2 as well.

We update the index and then immediately change the contents of the
file. This makes the diff think there are no changes, as the timestamp
of the file agrees with the cached data. This is however a bug, as the
file has obviously changed contents.

The test is a bit fragile, as it assumes that the index writing and the
following modification of the file happen in the same second, but it's
enough to show the issue.

Fix visibility of transaction symbol

path: remove unnecessary readdir_r usage

Arguably all uses of readdir_r are unnecessary, but in this case
especially so, as the directory handle only exists within this function,
so we don't race with anybody.

clone: fall back to copying when linking does not work

Fix in stransport_stream.c for usage of SecCopyErrorMessageString(), which is unavailable to iOS targets.

Binary diffs: store deltas in the diff structure, include binary data in diff callbacks

Fix leaks in tests/checkout/icase

Fix memory leak on windows in diriter.

Transaction.c did not include the visibility definition of its symbol
(that are in git2/transaction.h) and so was by default hidden.

Introduce a new binary diff callback to provide the actual binary
delta contents to callers.  Create this data from the diff contents
(instead of directly from the ODB) to support binary diffs including
the workdir, not just things coming out of the ODB.

Fixed some Secure Transport issues on OS X

travis: fail if we fail the push tests

We need to pass these options in order to have the credentials callback
set.

These tests were not being taken into consideration for the failure of
the test. They've been failing for a while now, but we hadn't noticed as
Travis was reporting the builds successful.

The read and write callbacks passed to SSLSetIOFuncs() have been
rewritten to match the implementation used on opensource.apple.com and
other open source projects like VLC.

This change also fixes a bug where the read callback could get into
an infinite loop when 0 bytes were read.

cred_helpers: Add 'const' qualifiers to git_cred_userpass_payload