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.