When computing a short OID we do this by first copying the
leading parts into the new OID structure and then setting the
trailing part to zero. In the case of the desired length being
`GIT_OID_HEXSZ - 1` we will call `memset` with an out of bounds
pointer and a length of 0. While this seems to cause no problems
for common platforms the C89 standard does not explicitly state
that calling `memset` with an out of bounds pointer and
length of 0 is valid.

Fix the potential issue by using the newly introduced
`git_oid__cpy_prefix` function.

This fixes an issue in Xcode 7.3 in objective-git where we get the error
"Include of non-modular header file in module". Not importing this
header again fixes the issue.

When parsing a section header we expect something along the
format of '[section "subsection"]'. When a section is
mal-formated and is entirely missing its quotation marks we catch
this case by observing that `strchr(line, '"') - strrchr(line,
'"') = NULL - NULL = 0` and error out. Unfortunately, the error
message is misleading though, as we state that we are missing the
closing quotation mark while we in fact miss both quotation
marks.

Improve the error message by explicitly checking if the first
quotation mark could be found and, if not, stating that quotation
marks are completely missing.

The first time may be due to memory fragmentation or just bad luck on a
32-bit system. When we hit the mmap error for the first time, free up
the unused windows and try again.

When looking up an abbreviated oid, show the actual (abbreviated) oid
the caller passed instead of a full (but ambiguously truncated) oid.

The inner packet may be split across multiple sideband packets.

ssh, ssh+git and git+ssh should all successfully build an SSH transport

Also moving var declarations to top of blocks to support bad old compilers

The parser now also supports digits, '-' and '.'. https://github.com/nodejs/http-parser/pull/276

treebuilder: don't try to verify submodules exist in the odb

Submodules don't exist in the objectdb and the code is making us try to
look for a blob with its commit id, which is obviously not going to
work.

Skip the test if the user wants to insert a submodule.

ssh: initialize libssh2

We should have been doing this, but it initializes itself upon first
use, which works as long as nobody's doing concurrent network
operations. Initialize it on our init to make sure it's not getting
initialized concurrently.

Expand OpenSSL and libssh2 thread safety documentation

test: make sure we retry the auth callback on all platforms

xdiff: fix memleak on error case

If the caller has provided bad authentication, give them another
apportunity to get it right until they give up. This brings WinHTTP in
line with the other transports.

We were missing this test on Windows, which meant we didn't notice that
we never fixed the single authentication attempt it tries, nor its wrong
return code.

Enable this for the unix platforms as well over HTTP. We previously were
doing it locally but disabled it on OS X due to issues with its sshd not
accepting password authentication.

Stricter object dependency checking during creation

Commit 3d1abc5a fixes a memory leak in the xdiff code. In the
process of upstreaming the fix it was pointed out by Johannes
Schindelin that there is another memory leak present (see [1]).

Fix the second memory leak by applying the upstream fix to our
code base.

[1]: http://thread.gmane.org/gmane.comp.version-control.git/287034

Few p_getaddrinfo fixes

Use legitimate (existing) object IDs in tests so that we have the
ability to turn on strict object validation when running tests.