Does not open submodule's URL when running it from outside submodule directory

[rhysd]

Repro

git clone https://github.com/rhysd/wain.git
cd wain/spec-test
git brws ./wasm-testsuite/f32.wast

Expected

Open https://github.com/WebAssembly/testsuite/blob/xxx/globals.wast

Actual

Opened https://github.com/rhysd/wain/blob/xxx/spec-test/wasm-testsuite/f32.wast

Description

git-brws does not consider submodule. When opening file, running git command within the file's directory would solve this.