In your home directory, you should edit .bash_profile if you have Git for Windows 2.21.0 or later (as of this writing).
You could direct .bash_profile to just source .bashrc, but if something happens to your .bash_profile, then it will be unclear why your .bashrc is again not working.
I put all my aliases and other environment stuff in .bash_profile, and I also added this line:
echo "Sourcing ~/.bash_profile - this version of Git Bash doesn't use .bashrc"
And THEN, in .bashrc I have
echo "This version of Git Bash doesn't use .bashrc. Use .bash_profile instead"
(Building on @harsel's response. I woulda commented, but I have no points yet.)