Currently, when choosing the bash shell option in Windows, the following is the command:

C:\Program Files\Git\bin\bash.EXE --noprofile --norc -e -o pipefail {0}

Note that C:\Program Files\Git\bin is already in PATH.

MSYS2 is now included in the Windows image, and the Git bash shell is based on it.

If the path was removed from the command, as below, people could switch to the MSYS2 bash shell by simply adjusting PATH.

bash.exe --noprofile --norc -e -o pipefail {0}

Discussions about MSYS2 have been going on for a while, I know some people have asked for a way to use its bash shell...

EDIT: See issue #460, as custom shells are not working.