Fix a couple of CommentAsPatch issues. (#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

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

* 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

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

* 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.go

@@ -125,30 +125,39 @@ var hunkRegex = regexp.MustCompile(`^@@ -(?P<beginOld>[0-9]+)(,(?P<endOld>[0-9]+
 
 const cmdDiffHead = "diff --git "
 
-func isHeader(lof string) bool {
-	return strings.HasPrefix(lof, cmdDiffHead) || strings.HasPrefix(lof, "---") || strings.HasPrefix(lof, "+++")
+func isHeader(lof string, inHunk bool) bool {
+	return strings.HasPrefix(lof, cmdDiffHead) || (!inHunk && (strings.HasPrefix(lof, "---") || strings.HasPrefix(lof, "+++")))
 }
 
 // CutDiffAroundLine cuts a diff of a file in way that only the given line + numberOfLine above it will be shown
 // it also recalculates hunks and adds the appropriate headers to the new diff.
 // Warning: Only one-file diffs are allowed.
-func CutDiffAroundLine(originalDiff io.Reader, line int64, old bool, numbersOfLine int) string {
+func CutDiffAroundLine(originalDiff io.Reader, line int64, old bool, numbersOfLine int) (string, error) {
 	if line == 0 || numbersOfLine == 0 {
 		// no line or num of lines => no diff
-		return ""
+		return "", nil
 	}
+
 	scanner := bufio.NewScanner(originalDiff)
 	hunk := make([]string, 0)
+
 	// begin is the start of the hunk containing searched line
 	// end is the end of the hunk ...
 	// currentLine is the line number on the side of the searched line (differentiated by old)
 	// otherLine is the line number on the opposite side of the searched line (differentiated by old)
 	var begin, end, currentLine, otherLine int64
 	var headerLines int
+
+	inHunk := false
+
 	for scanner.Scan() {
 		lof := scanner.Text()
 		// Add header to enable parsing
-		if isHeader(lof) {
+
+		if isHeader(lof, inHunk) {
+			if strings.HasPrefix(lof, cmdDiffHead) {
+				inHunk = false
+			}
 			hunk = append(hunk, lof)
 			headerLines++
 		}
@@ -157,6 +166,7 @@ func CutDiffAroundLine(originalDiff io.Reader, line int64, old bool, numbersOfLi
 		}
 		// Detect "hunk" with contains commented lof
 		if strings.HasPrefix(lof, "@@") {
+			inHunk = true
 			// Already got our hunk. End of hunk detected!
 			if len(hunk) > headerLines {
 				break
@@ -213,15 +223,19 @@ func CutDiffAroundLine(originalDiff io.Reader, line int64, old bool, numbersOfLi
 			}
 		}
 	}
+	err := scanner.Err()
+	if err != nil {
+		return "", err
+	}
 
 	// No hunk found
 	if currentLine == 0 {
-		return ""
+		return "", nil
 	}
 	// headerLines + hunkLine (1) = totalNonCodeLines
 	if len(hunk)-headerLines-1 <= numbersOfLine {
 		// No need to cut the hunk => return existing hunk
-		return strings.Join(hunk, "\n")
+		return strings.Join(hunk, "\n"), nil
 	}
 	var oldBegin, oldNumOfLines, newBegin, newNumOfLines int64
 	if old {
@@ -256,5 +270,5 @@ func CutDiffAroundLine(originalDiff io.Reader, line int64, old bool, numbersOfLi
 	// construct the new hunk header
 	newHunk[headerLines] = fmt.Sprintf("@@ -%d,%d +%d,%d @@",
 		oldBegin, oldNumOfLines, newBegin, newNumOfLines)
-	return strings.Join(newHunk, "\n")
+	return strings.Join(newHunk, "\n"), nil
 }