Can't parse config file if the encoding is UTF-8 BOM

What steps will reproduce the problem?

1. Change encoding of .gitmodules file from UTF-8 to UTF-8 BOM (See https://gitlab.com/YueLinHo/tortoisegit/commits/utf8_cfg_file branch as testing case)
2. Invoke Submodule Update dialog

What is the expected output? What do you see instead?

Expect: shows all submodules in list
Instead: empty list

What version of TortoiseGit and Git are you using? On what operating system?

TortoiseGit 2.5.7.0
(20180127-b2d00f87; C:\Program Files\TortoiseGit\bin)

git version 2.15.1.windows.2
(C:\Program Files\Git\bin; C:\Program Files\Git\mingw64; C:\Program Files\Git\mingw64\etc\gitconfig; C:\ProgramData\Git\config)

Please provide any additional information below.

TortoiseGit 2.5.0 is good.

I can trace how it failed to parse, and it's within the libgit2 code.

For testing case, also see https://github.com/YueLinHo/TortoiseGit/tree/utf8_cfg_file branch.