Commit message autocompletion doesn't work with UTF-8 encoded files

What steps will reproduce the problem?

1. Write some C# code that has a method with an umlaut character (ä, ö, ü) in it's name, say public IQueryable<Gebäude> GetGebäude() => Entities.Gebäude;
2. Open the commit dialog and type "Get", then hit CTRL-Space to open the autocomplete dropdown

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

Expected output: GetRäume is in the list
Actual output: GetRäume is missing (among what seems to be all other names with an umlaut in them)

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

• TortoiseGit 2.2.0.0 (C:\Devel\TortoiseGit\bin)
• git version 2.9.0.windows.1 (C:\Devel\Git\bin; C:\Devel\Git\mingw64; C:\Devel\Git\mingw64\etc\gitconfig; C:\ProgramData\Git\config)
• Windows 7 Enterprise x64

Please provide any additional information below.

Using TortoiseGitProc's autotexttest command I can see that the current regex for C# files does indeed match the names with umlauts in them, but then something between that regex and the autocomplete dropdown must choke on them.