Fix diff path unquoting

services/gitdiff/gitdiff.go whereby there it assumed that the path would
always be quoted on both sides

This PR simplifies the code here and uses fmt.Fscanf to parse the
strings as necessary.

Fix go-gitea#12546

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

services/gitdiff/gitdiff.go

@@ -18,7 +18,6 @@ import (
 	"os/exec"
 	"regexp"
 	"sort"
-	"strconv"
 	"strings"
 
 	"code.gitea.io/gitea/models"
@@ -570,40 +569,28 @@ func ParsePatch(maxLines, maxLineCharacters, maxFiles int, reader io.Reader) (*D
 				break
 			}
 
-			var middle int
-
 			// Note: In case file name is surrounded by double quotes (it happens only in git-shell).
 			// e.g. diff --git "a/xxx" "b/xxx"
-			hasQuote := line[len(cmdDiffHead)] == '"'
-			if hasQuote {
-				middle = strings.Index(line, ` "b/`)
+			var a string
+			var b string
+
+			rd := strings.NewReader(line[len(cmdDiffHead):])
+			char, _ := rd.ReadByte()
+			_ = rd.UnreadByte()
+			if char == '"' {
+				fmt.Fscanf(rd, "%q ", &a)
 			} else {
-				middle = strings.Index(line, " b/")
+				fmt.Fscanf(rd, "%s ", &a)
 			}
-
-			beg := len(cmdDiffHead)
-			a := line[beg+2 : middle]
-			b := line[middle+3:]
-
-			if hasQuote {
-				// Keep the entire string in double quotes for now
-				a = line[beg:middle]
-				b = line[middle+1:]
-
-				var err error
-				a, err = strconv.Unquote(a)
-				if err != nil {
-					return nil, fmt.Errorf("Unquote: %v", err)
-				}
-				b, err = strconv.Unquote(b)
-				if err != nil {
-					return nil, fmt.Errorf("Unquote: %v", err)
-				}
-				// Now remove the /a /b
-				a = a[2:]
-				b = b[2:]
-
+			char, _ = rd.ReadByte()
+			_ = rd.UnreadByte()
+			if char == '"' {
+				fmt.Fscanf(rd, "%q", &b)
+			} else {
+				fmt.Fscanf(rd, "%s", &b)
 			}
+			a = a[2:]
+			b = b[2:]
 
 			curFile = &DiffFile{
 				Name:      b,