Add Mutual TLS support to GitRepository

[stefanprodan]

Update fluxcd/pkg/git to v0.26.0 which includes:

• [v5] plumbing: support mTLS for HTTPS protocol go-git/go-git#1510
• Support ClientCert and ClientKey (mTLS) configuration in Git pkg#904

Closes: #1761

Git HTTPS Mutual TLS authentication

To authenticate towards a Git repository over HTTPS using mutual TLS, the referenced Secret's .data should contain the following keys:

• tls.crt and tls.key, to specify the client certificate and private key used
  for TLS client authentication. These must be used in conjunction, i.e.
  specifying one without the other will lead to an error.
• ca.crt, to specify the CA certificate used to verify the server, which is
  required if the server is using a self-signed certificate.

---
apiVersion: v1
kind: Secret
metadata:
  name: https-tls-certs
  namespace: default
type: Opaque
data:
  tls.crt: <BASE64>
  tls.key: <BASE64>
  ca.crt: <BASE64>