xdiff fixes

xdiff craps the bed on large files.  Treat very large files as binary,
so that it doesn't even have to try.

Refactor our merge binary handling to better match git.git, which
looks for a NUL in the first 8000 bytes.

refdb and odb backends must provide `free` function

As refdb and odb backends can be allocated by client code, libgit2
can’t know whether an alternative memory allocator was used, and thus
should not try to call `git__free` on those objects.

Instead, odb and refdb backend implementations must always provide
their own `free` functions to ensure memory gets freed correctly.

portability: use `CHECK_FUNCTION_EXISTS` for checking whether functions exist...

openssl: don't try to teardown an unconnected SSL context

SSL_shutdown() does not like it when we pass an unitialized ssl context
to it. This means that when we fail to connect to a host, we hide the
error message saying so with OpenSSL's indecipherable error message.

Include custom HTTP headers

win32::longpath: don't print path

net: add tests against badssl.com

These provide bad X.509 certificates, which we should refuse to connect
to by default.

Fix binary diffs

git expects an empty line after the binary data:

literal X
...binary data...
<empty_line>

The last literal block of the generated patches were not containing the required empty line. Example:

	diff --git a/binary_file b/binary_file
	index 3f1b3f9098131cfecea4a50ff8afab349ea66d22..86e5c1008b5ce635d3e3fffa4434c5eccd8f00b6 100644
	GIT binary patch
	literal 8
	Pc${NM&PdElPvrst3ey5{

	literal 6
	Nc${NM%g@i}0ssZ|0lokL
	diff --git a/binary_file2 b/binary_file2
	index 31be99be19470da4af5b28b21e27896a2f2f9ee2..86e5c1008b5ce635d3e3fffa4434c5eccd8f00b6 100644
	GIT binary patch
	literal 8
	Pc${NM&PdElPvrst3ey5{

	literal 13
	Sc${NMEKbZyOexL+Qd|HZV+4u-

git apply of that diff results in:

	error: corrupt binary patch at line 9: diff --git a/binary_file2 b/binary_file2
	fatal: patch with only garbage at line 10

The proper formating is:

	diff --git a/binary_file b/binary_file
	index 3f1b3f9098131cfecea4a50ff8afab349ea66d22..86e5c1008b5ce635d3e3fffa4434c5eccd8f00b6 100644
	GIT binary patch
	literal 8
	Pc${NM&PdElPvrst3ey5{

	literal 6
	Nc${NM%g@i}0ssZ|0lokL
	diff --git a/binary_file2 b/binary_file2
	index 31be99be19470da4af5b28b21e27896a2f2f9ee2..86e5c1008b5ce635d3e3fffa4434c5eccd8f00b6 100644
	GIT binary patch
	literal 8
	Pc${NM&PdElPvrst3ey5{

	literal 13
	Sc${NMEKbZyOexL+Qd|HZV+4u-

If the header doesn't look like a header (e.g. if it doesn't have a ":"
or if it has newlines), report "custom HTTP header '%s' is malformed".

If the header has the same name as a header already set by libgit2 (e.g.
"Host"), report "HTTP header '%s' is already set by libgit2".

submodule: plug a few leaks

win32: give better error message on long paths

Win32 Reserved names: don't reserve names outside the working directory

Keep config comments in the same place as git

Check that the repository directory is beneath the workdir before
adding it to the list of reserved paths.  If it is not, then there
is no possibility of checking out files into it, and it should not
be a reserved word.

This is a particular problem with submodules where the repo directory
may be in the super's .git directory.

Test an initial submodule update, where we are trying to checkout
the submodule for the first time, and placing a file within the
submodule working directory with the same name as the submodule
(and consequently, the same name as the repository itself).

When there is a comment at the end of a section, git keeps it there,
while we write the new variable right at the end.

Keep comments buffered and dump them when we're going to output a
variable or section, or reach EOF. This puts us in line with the config
files which git produces.

pkg-config: fix directory references in libgit2.pc

Before:
libdir=/usr//usr/lib64
includedir=/usr//usr/include

After:
libdir=/usr/lib64
includedir=/usr/include

(note the duplication of /usr in the before case)