The git.exe from Github for windows is located in a path like C:\Users\<username>\AppData\Local\GitHub\PortableGit_<numbersandletters>\bin\git.exe1 You have to replace <username> and <numbersandletters> to the actual situation on your system.
In Android Studio you can specify the path to the Git executable at File->Settings...->Version Control->Git->Path to Git executable. Here you have to include the actual executable name. As an example, in my case the actual path is: C:\Users\dennis\AppData\Local\GitHub\PortableGit_69703d1db91577f4c666e767a6ca5ec50a48d243\bin\git.exe
Edit: Last git update has put the git.exe file in cmd\ folder instead of bin\ . so now the actual path will be as suggested in the comment below by al3xAndr3w.
C:\Users\<username>\AppData\Local\GitHub\PortableGit_<numbersandletters>\cmd\git.exe