x/tools/internal/diff: non-minimal diff

[adonovan]

The x/tools diff algorithm reports correct but non-minimal diffs even for the trivial case where a new line is inserted in the middle of a two-line file. For example:

	got := "aaa\nccc\n"
	want := "aaa\nbbb\nccc\n"
	delta := diff.Unified("got", "want", got, want)

GNU diff -u:

--- want
+++ got
@@ -1,2 +1,3 @@
 aaa
+bbb
 ccc

diff.Unified:

--- got
+++ want
@@ -1,2 +1,3 @@
-aaa
+aaa
+bbb
 ccc

We should match the GNU diff behavior in cases this simple.

[gopherbot]

Change https://go.dev/cl/489695 mentions this issue: gopls/diff/unified: remove redundant information

[gopherbot]

Change https://go.dev/cl/499377 mentions this issue: internal/diff: fix LineEdits bug in slow path