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.

refspec: No remote tracking ref from a fetchspec-less remote

There is a bug in building the linked list of line records in the
diff iterator and also an off by one element error in the hunk
counts.  This fixes both of these, adds some test data with more
complex sets of hunk and line diffs to exercise this code better.

Add git_repository_hashfile to hash with filters

odb_pack: try lookup before refreshing packs

This reduces the rate of syscalls for the common case of sequences of
object reads from the same pack.

Best of 5 timings for libgit2_clar before this patch:
real    0m5.375s
user    0m0.392s
sys     0m3.564s

After applying this patch:
real    0m5.285s
user    0m0.356s
sys     0m3.544s

0.6% improvement in system time.
9.2% improvement in user time.
1.7% improvement in elapsed time.

Confirmed a 0.6% reduction in number of system calls with strace.

Expect greater improvement for graph-traversal with large packs.