feat: Validate Git tokens before consuming them (#5167)

* feat: Validate Git tokens before consuming them

This works the exact same way that the Git credential manager does. It ensures the user token is valid before returning it to the client.

It's been manually tested on GitHub, GitLab, and BitBucket.

* Fix requested changes

cli/deployment/config_test.go

@@ -179,6 +179,7 @@ func TestConfig(t *testing.T) {
 			"CODER_GITAUTH_0_CLIENT_SECRET": "secret",
 			"CODER_GITAUTH_0_AUTH_URL":      "https://auth.com",
 			"CODER_GITAUTH_0_TOKEN_URL":     "https://token.com",
+			"CODER_GITAUTH_0_VALIDATE_URL":  "https://validate.com",
 			"CODER_GITAUTH_0_REGEX":         "github.com",
 			"CODER_GITAUTH_0_SCOPES":        "read write",
 			"CODER_GITAUTH_0_NO_REFRESH":    "true",
@@ -200,6 +201,7 @@ func TestConfig(t *testing.T) {
 				ClientSecret: "secret",
 				AuthURL:      "https://auth.com",
 				TokenURL:     "https://token.com",
+				ValidateURL:  "https://validate.com",
 				Regex:        "github.com",
 				Scopes:       []string{"read", "write"},
 				NoRefresh:    true,

cli/gitaskpass.go

@@ -62,7 +62,7 @@ func gitAskpass() *cobra.Command {
 					if err != nil {
 						continue
 					}
-					cmd.Printf("\nYou've been authenticated with Git!\n")
+					cmd.Printf("You've been authenticated with Git!\n")
 					break
 				}
 			}