Convert git commit summary to valid UTF8. (#28356) (#28358)

Backport #28356 by @darrinsmart

The summary string ends up in the database, and (at least) MySQL &
PostgreSQL require valid UTF8 strings.

Fixes #28178

Co-authored-by: darrinsmart <darrin@djs.to>
Co-authored-by: Darrin Smart <darrin@filmlight.ltd.uk>

modules/git/commit.go

@@ -43,8 +43,9 @@ func (c *Commit) Message() string {
 }
 
 // Summary returns first line of commit message.
+// The string is forced to be valid UTF8
 func (c *Commit) Summary() string {
-	return strings.Split(strings.TrimSpace(c.CommitMessage), "\n")[0]
+	return strings.ToValidUTF8(strings.Split(strings.TrimSpace(c.CommitMessage), "\n")[0], "?")
 }
 
 // ParentID returns oid of n-th parent (0-based index).