Fix handling of paths with white spaces on diff parsing

[Nfsaavedra]

Context

While I was using pygit2 I got a segmentation fault when trying to use pygit2.Repository.apply with a diff obtained by using parse_diff. The diff contained a removal of a file with spaces on its path and the path was not quoted:

diff --git a/.run/Run IDE with Plugin.run.xml b/.run/Run IDE with Plugin.run.xml

What I found was that when I transformed the diff obtained from parse_diff back to a string, the previous line was changed to:

diff --git a/.run/Run IDE with Plugin.run.xml /dev/null

For some reason that I still don't fully understand, but that makes some sense to me, the /dev/null created a segmentation fault on the apply. I looked into pygit2 and found out that the problem probably was on libgit2.

Changes

I was able to debug and find out that the function header_path_len was the culprit. Since previously this function stopped on a white space if the path is not quoted, then the header I gave as example would be incorrectly parsed. Given this, I changed the function so it only stops when it reaches b/ or "b/. This still has a problem if a folder called b (white space followed by b) or "b is used but it seems pretty unlikely.

Tests

I created a test for delete mode since a test for create mode already existed. However I found it weird that the tests expected that on create mode the result for the old file path was NULL. The reason I found it weird is that for "normal" paths, i.e. paths without spaces, the result was the path of the file instead of NULL. For this reason, I also changed the test for create mode to go accordingly with the behavior of "normal" paths.

[arroz]

We have a patch waiting for what I think is the same problem: #6295 I had missed the review there, just committed the suggested code.

[Nfsaavedra]

We have a patch waiting for what I think is the same problem: #6295 I had missed the review there, just committed the suggested code.

Oops forgot to check that, thanks! I'll close this PR