Instead of having it set in a different place from every other callback,
put it the main structure. This removes some state from the remote and
makes it behave more like clone, where the constructors are passed via
the options.

Add a prune setting in the fetch options to allow to fall back to the
configuration (the default) or to set it on or off.

As a first step in removing the repository-saving logic, don't allow
chaning the url or push url from a remote object, but change the
configuration on the configuration immediately.

Having the setting be different from calling its actions was not a great
idea and made for the sake of the wrong convenience.

Instead of that, accept either fetch options, push options or the
callbacks when dealing with the remote. The fetch options are currently
only the callbacks, but more options will be moved from setters and
getters on the remote to the options.

This does mean passing the same struct along the different functions but
the typical use-case will only call git_remote_fetch() or
git_remote_push() and so won't notice much difference.

The push object knows which remote it's associated with, and therefore
does not need to keep its own copy of the callbacks stored in the
remote.

Remove the copy and simply access the callbacks struct within the
remote.

Centralizing all IO buffer size values

Stash apply

Allow the baseline to be specified as an index, so that users
need not write their index to a tree just to checkout with that
as the baseline.

Adjusting stream buffer size to 64KB

64K is optimal buffer size per https://technet.microsoft.com/en-us/library/cc938632.aspx

Android build doesn't need deps/regex

Ensure frameworks are mentioned in libgit2.pc

When building on Mac OS X, the `CoreFoundation` and `Security` frameworks where missing from `Libs.private` in the generated `libgit2.pc` file.

deps/regex was included in Android build because Android NDK 4 has
a packaging bug and doesn't have the regular expression functions defined
in its libc.so. The bug has been fixed in subsequent Android NDK releases.

If it is still necessary to work around the bug in Android NDK 4, we
should consider to use an option like ANDROID_NDK_RELEASE or
ANDROID_NDK_RELEASE_NUM.

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.