If you need to run tmux in git bash, this worked for me:
1. Installs msys2 (which includes pacman package manager) # can be done separately from your git installation
2. pacman -S tmux # install tmux to msys2 within msys2
3. Copy tmux.exe and msys-event-2-1-7*.dll from msys2 to C:\Program Files\Git\usr\bin # may be a different version msys-event
4. Uninstall msys2! This was important to me because I found edge cases where msys2 did not honor directory links as "not msys files" and it borked things. YMMV