run-tests.py: fix diff output when test-foo.out doesn't exist.
Previously, if test-foo.out doesn't exist and test-foo outputs
something, the first line of the diff will look like this:
-+changeset: 0:0acdaf898367
(note the leading "-")
adding foo
>>> two diffs showing three added lines <<<
diff -r 540c40a65b78 foo
--- a/foo
+++ b/foo
@@ -1,2 +1,5 @@ hello world
+
hello world
+
goodbye world
+
diff -r 540c40a65b78 foo
--- a/foo
+++ b/foo
@@ -1,2 +1,5 @@ hello world
+
hello world
+
goodbye world
+
>>> no diffs <<<
>>> four diffs showing added space first on the first line <<<
diff -r 540c40a65b78 foo
--- a/foo
+++ b/foo
@@ -1,2 +1,2 @@ hello world
-hello world
+ hello world
goodbye world
diff -r 540c40a65b78 foo
--- a/foo
+++ b/foo
@@ -1,2 +1,2 @@ hello world
-hello world
+ hello world
goodbye world
diff -r 540c40a65b78 foo
--- a/foo
+++ b/foo
@@ -1,2 +1,2 @@ hello world
-hello world
+ hello world
goodbye world
diff -r 540c40a65b78 foo
--- a/foo
+++ b/foo
@@ -1,2 +1,2 @@ hello world
-hello world
+ hello world
goodbye world
>>> two diffs showing space appended to the first line <<<
diff -r 540c40a65b78 foo
--- a/foo
+++ b/foo
@@ -1,2 +1,2 @@ hello world
-hello world
+hello world
goodbye world
diff -r 540c40a65b78 foo
--- a/foo
+++ b/foo
@@ -1,2 +1,2 @@ hello world
-hello world
+hello world
goodbye world
>>> no diffs <<<
>>> four diffs showing space inserted into "goodbye" <<<
diff -r 540c40a65b78 foo
--- a/foo
+++ b/foo
@@ -1,2 +1,2 @@ hello world
hello world
-goodbye world
+good bye world
diff -r 540c40a65b78 foo
--- a/foo
+++ b/foo
@@ -1,2 +1,2 @@ hello world
hello world
-goodbye world
+good bye world
diff -r 540c40a65b78 foo
--- a/foo
+++ b/foo
@@ -1,2 +1,2 @@ hello world
hello world
-goodbye world
+good bye world
diff -r 540c40a65b78 foo
--- a/foo
+++ b/foo
@@ -1,2 +1,2 @@ hello world
hello world
-goodbye world
+good bye world
>>> two diffs showing changed whitespace amount in the last line <<<
diff -r 540c40a65b78 foo
--- a/foo
+++ b/foo
@@ -1,2 +1,2 @@ hello world
hello world
-goodbye world
+goodbye world
diff -r 540c40a65b78 foo
--- a/foo
+++ b/foo
@@ -1,2 +1,2 @@ hello world
hello world
-goodbye world
+goodbye world
>>> no diffs <<<
>>> four diffs showing added blank line w/horizontal space <<<
diff -r 540c40a65b78 foo
--- a/foo
+++ b/foo
@@ -1,2 +1,3 @@ hello world
hello world
+
goodbye world
diff -r 540c40a65b78 foo
--- a/foo
+++ b/foo
@@ -1,2 +1,3 @@ hello world
hello world
+
goodbye world
diff -r 540c40a65b78 foo
--- a/foo
+++ b/foo
@@ -1,2 +1,3 @@ hello world
hello world
+
goodbye world
diff -r 540c40a65b78 foo
--- a/foo
+++ b/foo
@@ -1,2 +1,3 @@ hello world
hello world
+
goodbye world