There has been discussion for a while about making some set of
the `giterr_set` type functions part of the public API for code
that is implementing new backends to libgit2.  This makes the
`giterr_set_str()` and `giterr_set_oom()` functions public.

checkout: add notification callback for skipped files

git_repository_hashfile: Only close file handle if we have a valid one

Otherwise this throws an exception on MFC based systems.

Signed-off-by: Sven Strickroth <email@cs-ware.de>

Checkout, reset and others enhancements

Allocate a buffer large enough to store the path plus the terminator
instead of letting readlink write beyond the end.

odb_pack: make sure to search all backends

last_found is the last packfile a wanted object was found in. Since
last_found is shared among all searching threads, it might changes while
we're searching. As suggested by @arrbee, put a copy on the stack to fix
the race condition.

refs: prevent locked refs from being enumerated

Fix #936

Defining the BOM as a string makes the array include the
NUL-terminator, which means that the memcpy is going to check for that
as well and thus never match for a nonempty file.

Define the array as three chars, which makes the size correct.

I don't think clone.c needs in #include dirent.h and it is not
portable, so let's just get rid of it.

Fix MSVC compilation warnings

pack: iterate objects in offset order

Compute the ordering on demand and persist until the index is freed.

The winhttp branch was based on a version before these existed, so the
build broke on Windows.

http: use WinHTTP on Windows

Wondows has its own HTTP library. Use that one when possible instead of
our own.

As we don't depend on them anymore, remove the http-parser library from
the Windows build, as well as the search for OpenSSL.

Fix error text s/buffer too long/buffer too short/

Fix problems in diff iterator record chaining

The last change tweaked the way we use the hunk_curr pointer
during iteration, but failed to reset the value back to NULL
when switching files.