Refactor parseSignatureFromCommitLine (#29054)

Replace #28849. Thanks to @yp05327 for the looking into the problem.
Fix #28840

The old behavior of newSignatureFromCommitline is not right. The new
parseSignatureFromCommitLine:

1. never fails
2. only accept one format (if there is any other, it could be easily added)

And add some tests.

modules/git/signature.go

@@ -4,7 +4,46 @@
 
 package git
 
-const (
-	// GitTimeLayout is the (default) time layout used by git.
-	GitTimeLayout = "Mon Jan _2 15:04:05 2006 -0700"
+import (
+	"strconv"
+	"strings"
+	"time"
+
+	"code.gitea.io/gitea/modules/log"
 )
+
+// Helper to get a signature from the commit line, which looks like:
+//
+//	full name <user@example.com> 1378823654 +0200
+//
+// Haven't found the official reference for the standard format yet.
+// This function never fails, if the "line" can't be parsed, it returns a default Signature with "zero" time.
+func parseSignatureFromCommitLine(line string) *Signature {
+	sig := &Signature{}
+	s1, sx, ok1 := strings.Cut(line, " <")
+	s2, s3, ok2 := strings.Cut(sx, "> ")
+	if !ok1 || !ok2 {
+		sig.Name = line
+		return sig
+	}
+	sig.Name, sig.Email = s1, s2
+
+	if strings.Count(s3, " ") == 1 {
+		ts, tz, _ := strings.Cut(s3, " ")
+		seconds, _ := strconv.ParseInt(ts, 10, 64)
+		if tzTime, err := time.Parse("-0700", tz); err == nil {
+			sig.When = time.Unix(seconds, 0).In(tzTime.Location())
+		}
+	} else {
+		// the old gitea code tried to parse the date in a few different formats, but it's not clear why.
+		// according to public document, only the standard format "timestamp timezone" could be found, so drop other formats.
+		log.Error("suspicious commit line format: %q", line)
+		for _, fmt := range []string{ /*"Mon Jan _2 15:04:05 2006 -0700"*/ } {
+			if t, err := time.Parse(fmt, s3); err == nil {
+				sig.When = t
+				break
+			}
+		}
+	}
+	return sig
+}