fix(git): fix Windows path separator mismatch in TestSetUserNative

fileContent() always joins folder+"/"+file, regardless of OS, but the
test built its mock's expected key with filepath.Join, which uses
backslashes on Windows. That mismatch made the mock call unexpected on
the windows-latest CI runner. Build the expected key the same way
fileContent() does.

Author: claude

src/segments/git_test.go

@@ -2368,7 +2368,7 @@ func TestSetUserNative(t *testing.T) {
 	// no RunCommand expectation at all, proves setUser reads the local
 	// config without ever spawning git.
 	env := new(mock.Environment)
-	env.On("FileContent", filepath.Join(gitDir, "config")).Return(string(configData))
+	env.On("FileContent", gitDir+"/config").Return(string(configData))
 
 	g := &Git{mainSCMDir: gitDir}
 	g.Init(options.Map{}, env)