Include custom HTTP headers

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