Use ssh user specified in the config for Git auth

src/git/git.go

 		return nil, err
 	}
 
-	return &gitssh.PublicKeys{User: "git", Signer: signer}, nil
+	return &gitssh.PublicKeys{User: user, Signer: signer}, nil
 }
 
 // clone clones a Git repository into a given path, using a given auth.