Check for final \t in --- and +++ line in diff

On some occasion (apparently when there is a space in the file name) ,
git add a tab at the '---’ and '+++’ lines output by git diff, making
the newer magit-wash-diff failed to correctly parse this line.
Add test for washing diff in the status buffer.

3 test added: one for staged and modified file with space in their name,
one for unstaged modified file, and one for unstaged modified file with
space in their name.

Those test pass, but not before ae9a178cf05e1eb4e87c58ff86a515f4a24f7f69
Merge pull request #1398 from vanicat/t/tab-in-diff

A fix for a quirks in git diff output