Run 'go fix' to modernize code (#69)

This only updates to Go 1.21 baseline, which is what the module still
specifies as the minimum version. I'll need to run this again when
moving to Go 1.25 or 1.26 as minimum.

gitdiff/patch_header.go

@@ -186,9 +186,9 @@ func ParsePatchHeader(header string, options ...PatchHeaderOption) (*PatchHeader
 	}
 
 	var firstLine, rest string
-	if idx := strings.IndexByte(header, '\n'); idx >= 0 {
-		firstLine = header[:idx]
-		rest = header[idx+1:]
+	if before, after, ok := strings.Cut(header, "\n"); ok {
+		firstLine = before
+		rest = after
 	} else {
 		firstLine = header
 		rest = ""
@@ -362,8 +362,8 @@ func parseHeaderMail(mailLine string, r io.Reader, opts patchHeaderOptions) (*Pa
 
 	h := &PatchHeader{}
 
-	if strings.HasPrefix(mailLine, mailHeaderPrefix) {
-		mailLine = strings.TrimPrefix(mailLine, mailHeaderPrefix)
+	if after, ok := strings.CutPrefix(mailLine, mailHeaderPrefix); ok {
+		mailLine = after
 		if i := strings.IndexByte(mailLine, ' '); i > 0 {
 			h.SHA = mailLine[:i]
 		}