Fix regex to support optional end line of old section in diff hunk (#5097)

+ Named groups in reges for easier group parsing

models/git_diff.go

@@ -273,7 +273,7 @@ func (diff *Diff) NumFiles() int {
 }
 
 // Example: @@ -1,8 +1,9 @@ => [..., 1, 8, 1, 9]
-var hunkRegex = regexp.MustCompile(`^@@ -([0-9]+),([0-9]+) \+([0-9]+)(,([0-9]+))? @@`)
+var hunkRegex = regexp.MustCompile(`^@@ -(?P<beginOld>[0-9]+)(,(?P<endOld>[0-9]+))? \+(?P<beginNew>[0-9]+)(,(?P<endNew>[0-9]+))? @@`)
 
 func isHeader(lof string) bool {
 	return strings.HasPrefix(lof, cmdDiffHead) || strings.HasPrefix(lof, "---") || strings.HasPrefix(lof, "+++")
@@ -311,21 +311,28 @@ func CutDiffAroundLine(originalDiff io.Reader, line int64, old bool, numbersOfLi
 			if len(hunk) > headerLines {
 				break
 			}
-			groups := hunkRegex.FindStringSubmatch(lof)
+			// A map with named groups of our regex to recognize them later more easily
+			submatches := hunkRegex.FindStringSubmatch(lof)
+			groups := make(map[string]string)
+			for i, name := range hunkRegex.SubexpNames() {
+				if i != 0 && name != "" {
+					groups[name] = submatches[i]
+				}
+			}
 			if old {
-				begin = com.StrTo(groups[1]).MustInt64()
-				end = com.StrTo(groups[2]).MustInt64()
+				begin = com.StrTo(groups["beginOld"]).MustInt64()
+				end = com.StrTo(groups["endOld"]).MustInt64()
 				// init otherLine with begin of opposite side
-				otherLine = com.StrTo(groups[3]).MustInt64()
+				otherLine = com.StrTo(groups["beginNew"]).MustInt64()
 			} else {
-				begin = com.StrTo(groups[3]).MustInt64()
-				if groups[5] != "" {
-					end = com.StrTo(groups[5]).MustInt64()
+				begin = com.StrTo(groups["beginNew"]).MustInt64()
+				if groups["endNew"] != "" {
+					end = com.StrTo(groups["endNew"]).MustInt64()
 				} else {
 					end = 0
 				}
 				// init otherLine with begin of opposite side
-				otherLine = com.StrTo(groups[1]).MustInt64()
+				otherLine = com.StrTo(groups["beginOld"]).MustInt64()
 			}
 			end += begin // end is for real only the number of lines in hunk
 			// lof is between begin and end