flipdiff: segfault when patches don't commute

[ralokt]

flipdiff currently segfaults with the following inputs:

fdsf1:

diff --git a/abc.txt b/abc.txt
index c0d0fb4..5b88d3e 100644
--- a/abc.txt
+++ b/abc.txt
@@ -1,2 +1,2 @@
-a
-b
+a-m
+b-y

fdsf2:

diff --git a/abc.txt b/abc.txt
index 5b88d3e..c36071c 100644
--- a/abc.txt
+++ b/abc.txt
@@ -1,2 +1,2 @@
-a-m
+a

and the following invocation:

flipdiff --debug fdsf{1,2}

Debug output:

Auto-determined -p0
1: -1
2: 1
@1: removed (-1)
@1: removed (-1)
@1: removed (-1)
@2: 0

I should note that these patches don't commute. However, such input can come from git diff (I edited the input to make the reproduction more minimal, but I discovered it looking at patches generated by git diff).