Fix SSH signing key path will be displayed in the pull request UI (#35381)

Closes #35361

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Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/git/key.go

@@ -3,13 +3,24 @@
 
 package git
 
+import "code.gitea.io/gitea/modules/setting"
+
 // Based on https://git-scm.com/docs/git-config#Documentation/git-config.txt-gpgformat
 const (
 	SigningKeyFormatOpenPGP = "openpgp" // for GPG keys, the expected default of git cli
 	SigningKeyFormatSSH     = "ssh"
 )
 
+// SigningKey represents an instance key info which will be used to sign git commits.
+// FIXME: need to refactor it to a new name, this name conflicts with the variable names for "asymkey.GPGKey" in many places.
 type SigningKey struct {
 	KeyID  string
 	Format string
 }
+
+func (s *SigningKey) String() string {
+	// Do not expose KeyID
+	// In case the key is a file path and the struct is rendered in a template, then the server path will be exposed.
+	setting.PanicInDevOrTesting("don't call SigningKey.String() - it exposes the KeyID which might be a local file path")
+	return "SigningKey:" + s.Format
+}