Commit 3d1abc5afc fixes a memory leak in the xdiff code. In the
process of upstreaming the fix it was pointed out by Johannes
Schindelin that there is another memory leak present (see [1]).
Fix the second memory leak by applying the upstream fix to our
code base.
[1]: http://thread.gmane.org/gmane.comp.version-control.git/287034
Upgrade xdiff to version used in core git 2.4.5 (0df0541).
Corrects an issue where an LF is added at EOF while applying
an unrelated change (ba31180), cleans up some unused code (be89977 and
e5b0662), and provides an improved callback to avoid leaking internal
(to xdiff) structures (467d348).
This also adds some additional functionality that we do not yet take
advantage of, namely the ability to ignore changes whose lines are
all blank (36617af).
