Configuration changes for handling multiple of the same sections

Case insensitive checkout improvements

If a multivar exists within two sections (of the same name)
then they should both be updated in a `set_multivar`.  Ensure
that this is the case.

When writing a configuration file, we want to take a lock on the
new file (eg, `config.lock`) before opening the configuration file
(`config`) for reading so that we can prevent somebody from changing
the contents underneath us.

Add a test that exposes a bug in config_write.
It is valid to have multiple separate headers for the same config section, but
config_write will exit after finding the first matching section in certain
situations.
This test proves that config_write will duplicate a variable that already
exists instead of overwriting it if the variable is defined under a duplicate
section header.

When updating a configuration file, we want to copy the old data
from the file to preserve comments and funny whitespace, instead
of writing it in some "canonical" format.  Thus, we keep a
pointer to the start of the line and the line length to preserve
these things we don't care to rewrite.

Previously we would try to be clever when writing the configuration
file and try to stop parsing (and simply copy the rest of the old
file) when we either found the value we were trying to write,
or when we left the section that value was in, the assumption being
that there was no more work to do.

Regrettably, you can have another section with the same name later
in the file, and we must cope with that gracefully, thus we read the
whole file in order to write a new file.

Now, writing a file looks even more than reading.  Pull the config
parsing out into its own function that can be used by both reading
and writing the configuration.

On Mac OS, `realpath` is deficient in determining the actual filename
on-disk as it will simply provide the string you gave it if that file
exists, instead of returning the filename as it exists.  Instead we
must read the directory entries for the parent directory to get the
canonical filename.

When checking out with a case-insensitive working directory, we
want to change the case of items in the working directory to
reflect changes that occured in the checkout target.  Diff now
has an option to break case-changing renames into delete/add.

This reverts commit 40d79154.

Ensure that on a case insensitive filesystem that we can checkout
into some folder 'FOLDER' that exists on disk, even if the target
of the checkout is a different case (eg 'folder').

On Windows, you might sloppily rewrite a file (or have a sloppy
text editor that does it for you) and accidentally change its
case.  (eg, "README" -> "readme").  Git ignores this accidental
case changing rename during checkout and will happily write the
new content to the file despite the name change.  We should, too.

Performance Improvements to Status on Windows

Added call to git_libgit2_shutdown()

Code restructured to better represent best practice when
using libgit2.

Don't search iconv in /opt/local

Since OpenSSL isn't used any more on OS X, there is no dependency
on any MacPorts library under /opt/local and there is no danger of
conflicts between MacPorts and system iconv. For this reason the
system iconv can always be used now.

Using FindFirstFile and FindNextFile in win32 allows us to
use the directory information that is returned, instead of
us having to get the file attributes all over again, which
is a distinct cost savings on win32.

The _next method shouldn't take a path pointer (and a path_len
pointer) as 100% of current users use the full path and ignore
the filename.

Plus let's add some docs and a unit test.

Introduce a new `git_path_diriter` that can iterate directories
efficiently for each platform.

[WIP/RFC] push: report the update plan to the caller

Fix some build warnings

Update documentation for API changes

Fixed unused warning in tests/rebase/merge.c

Added forgotten call to git_libgit2_shutdown() to the /examples/network/git2.c.