Add a test that verifies that we are able to parse patches which add a
new file that has spaces in its path.

Patches which contain exact renames only will not contain an actual diff
body, but only a list of files that were renamed. Thus, the patch header
is immediately followed by the terminating sequence "-- ". We currently
do not recognize this character sequence as a possible terminating
sequence. Add it and create a test to catch the failure.

The function `diff_parsed_alloc` allocates and initializes a
`git_diff_parsed` structure. This structure also contains diff options.
While we initialize its flags, we fail to do a real initialization of
its values. This bites us when we want to actually use the generated
diff as we do not se the option's version field, which is required to
operate correctly.

Fix the issue by executing `git_diff_init_options` on the embedded
struct.

In a diff, the shortest possible hunk with a modification (that is, no
deletion) results from a file with only one line with a single character
which is removed. Thus the following hunk

    @@ -1 +1 @@
    -a
    +

is the shortest valid hunk modifying a line. The function parsing the
hunk body though assumes that there must always be at least 4 bytes
present to make up a valid hunk, which is obviously wrong in this case.
The absolute minimum number of bytes required for a modification is
actually 2 bytes, that is the "+" and the following newline. Note: if
there is no trailing newline, the assumption will not be offended as the
diff will have a line "\ No trailing newline" at its end.

This patch fixes the issue by lowering the amount of bytes required.

The current logic of `git_diff_foreach` makes the assumption that all
diffs passed in are actually derived from generated diffs. With these
assumptions we try to derive the actual diff by inspecting either the
working directory files or blobs of a repository. This obviously cannot
work for diffs parsed from a file, where we do not necessarily have a
repository at hand.

Since the introduced split of parsed and generated patches, there are
multiple functions which help us to handle patches generically, being
indifferent from where they stem from. Use these functions and remove
the old logic specific to generated patches. This allows re-using the
same code for invoking the callbacks on the deltas.

Ensure that `git_patch_from_diff` can return the patch for parsed diffs,
not just generate a patch for a generated diff.

When showing copy information because we are duplicating contents,
for example, when performing a `diff --find-copies-harder -M100 -B100`,
then show copy from/to lines in a patch, and do not show context.
Ensure that we can also parse such patches.