Fix a couple of CommentAsPatch issues. (#14804) (#14820)

Backport #14804

* CutDiffAroundLine makes the incorrect assumption that `---` and `+++` always represent part of the header of a diff.

This PR adds a flag to its parsing to prevent this problem and adds a streaming parsing technique to CutDiffAroundLine using an io.pipe instead of just sending data to an unbounded buffer.

Fix #14711

* Handle unquoted comment patch files

When making comment patches unfortunately the patch does not always quote the filename
This makes the diff --git header ambiguous again.

This PR finally adds handling for ambiguity in to parse patch

Fix #14812

* Add in testing for no error

There is no way currently for CutDiffAroundLine in this test to cause an
error however, it should still be tested.

Signed-off-by: Andrew Thornton <art27@cantab.net>

modules/git/diff_test.go

@@ -23,8 +23,28 @@ const exampleDiff = `diff --git a/README.md b/README.md
 + cut off
 + cut off`
 
+const breakingDiff = `diff --git a/aaa.sql b/aaa.sql
+index d8e4c92..19dc8ad 100644
+--- a/aaa.sql
++++ b/aaa.sql
+@@ -1,9 +1,10 @@
+ --some comment
+--- some comment 5
++--some coment 2
++-- some comment 3
+ create or replace procedure test(p1 varchar2)
+ is
+ begin
+---new comment
+ dbms_output.put_line(p1);
++--some other comment
+ end;
+ /
+`
+
 func TestCutDiffAroundLine(t *testing.T) {
-	result := CutDiffAroundLine(strings.NewReader(exampleDiff), 4, false, 3)
+	result, err := CutDiffAroundLine(strings.NewReader(exampleDiff), 4, false, 3)
+	assert.NoError(t, err)
 	resultByLine := strings.Split(result, "\n")
 	assert.Len(t, resultByLine, 7)
 	// Check if headers got transferred
@@ -37,18 +57,50 @@ func TestCutDiffAroundLine(t *testing.T) {
 	assert.Equal(t, "+ Build Status", resultByLine[4])
 
 	// Must be same result as before since old line 3 == new line 5
-	newResult := CutDiffAroundLine(strings.NewReader(exampleDiff), 3, true, 3)
+	newResult, err := CutDiffAroundLine(strings.NewReader(exampleDiff), 3, true, 3)
+	assert.NoError(t, err)
 	assert.Equal(t, result, newResult, "Must be same result as before since old line 3 == new line 5")
 
-	newResult = CutDiffAroundLine(strings.NewReader(exampleDiff), 6, false, 300)
+	newResult, err = CutDiffAroundLine(strings.NewReader(exampleDiff), 6, false, 300)
+	assert.NoError(t, err)
 	assert.Equal(t, exampleDiff, newResult)
 
-	emptyResult := CutDiffAroundLine(strings.NewReader(exampleDiff), 6, false, 0)
+	emptyResult, err := CutDiffAroundLine(strings.NewReader(exampleDiff), 6, false, 0)
+	assert.NoError(t, err)
 	assert.Empty(t, emptyResult)
 
 	// Line is out of scope
-	emptyResult = CutDiffAroundLine(strings.NewReader(exampleDiff), 434, false, 0)
+	emptyResult, err = CutDiffAroundLine(strings.NewReader(exampleDiff), 434, false, 0)
+	assert.NoError(t, err)
 	assert.Empty(t, emptyResult)
+
+	// Handle minus diffs properly
+	minusDiff, err := CutDiffAroundLine(strings.NewReader(breakingDiff), 2, false, 4)
+	assert.NoError(t, err)
+
+	expected := `diff --git a/aaa.sql b/aaa.sql
+--- a/aaa.sql
++++ b/aaa.sql
+@@ -1,9 +1,10 @@
+ --some comment
+--- some comment 5
++--some coment 2`
+	assert.Equal(t, expected, minusDiff)
+
+	// Handle minus diffs properly
+	minusDiff, err = CutDiffAroundLine(strings.NewReader(breakingDiff), 3, false, 4)
+	assert.NoError(t, err)
+
+	expected = `diff --git a/aaa.sql b/aaa.sql
+--- a/aaa.sql
++++ b/aaa.sql
+@@ -1,9 +1,10 @@
+ --some comment
+--- some comment 5
++--some coment 2
++-- some comment 3`
+
+	assert.Equal(t, expected, minusDiff)
 }
 
 func BenchmarkCutDiffAroundLine(b *testing.B) {
@@ -69,7 +121,7 @@ func ExampleCutDiffAroundLine() {
  Docker Pulls
 + cut off
 + cut off`
-	result := CutDiffAroundLine(strings.NewReader(diff), 4, false, 3)
+	result, _ := CutDiffAroundLine(strings.NewReader(diff), 4, false, 3)
 	println(result)
 }