CI on X86_64 Windows fails due to bad --git-dir

[rhysd]

Job: https://ci.appveyor.com/project/rhysd/git-brws/branch/master/job/2rm5eqvclqhw653f

This failure is only on CI (both Appveyor and GitHub Actions). It is not reproducible on my local Windows machine. All tests passed on my local machine.

.canonicalize() uses \\?... path (extended path length) but it seems not supported by --git-dir. On Windows, --git-dir is not a file path actually. It is a kind of slash-path:

> git rev-parse --absolute-git-dir
D:/a/git-brws/git-brws/.git

\\?\D:\a\git-brws\git-brws\.git for --git-dir seems not available on some version of Windows. Following error occurs at running rev-parse --abbrev-ref --symbolic @{u} command:

fatal: not a git repository: '\\?\C:\git-brws\.git'

Actually it was OK on my local Windows machine to use the \\?... path as far as I tried manually.