As long as:
git is in your $PATH 
- git subrepo 
.rc file has been added to your .bashrc (source /path/to/git-subrepo/.rc' >> ~/.bashrc, done from a git bash session) 
You can use a regular git-bash shell, and start typing git subrepo commands.
The "shell startup configuration" is the .rc file which is where you have cloned the repository (in /c/path/to/your/git-subrepo/clone within a git bash session)
If you clone in C:\Users\Me\git-subrepo, you would need to type in a bash session:
source /c/Users/Me/git-subrepo/.rc' >> ~/.bashrc
The OP wiyosaya did (in the comments):
- create the .bashrc: 
touch ~/.bashrc 
- add the missing line: 
echo source /c/GitHub/git-subrepo/.rc >> ~/.bashrc