The diff implementation is different than `diff`

[hannesm]

If we take file1:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25

and file2:

1
2
3
4
5
6
7
8
9
hi
11
12
13
14
15
16
17
18
19
hi
21
22
23
24
25

the diff generated by normal diff -u is:

--- hunks.old
+++ hunks.new
@@ -7,7 +7,7 @@
 7
 8
 9
-10
+hi
 11
 12
 13
@@ -17,7 +17,7 @@
 17
 18
 19
-20
+hi
 21
 22
 23

but our diff generates the following:

--- hunks.old
+++ hunks.new
@@ -10,16 +10,16 @@
-10
-11
-12
-13
-14
-15
-16
-17
-18
-19
-20
-21
-22
-23
-24
-25
+hi
+11
+12
+13
+14
+15
+16
+17
+18
+19
+hi
+21
+22
+23
+24
+25

-> missing context (the three lines before the change), and we could emit multiple hunks...

[hannesm]

Please note that I don't have any motivation to work on this, but if someone needs the diff implementation and is keen to work on it, please do.