git mv mycoolclass.cs myCoolClass.cs
# Short answer:
git mv filename new_filename
or
git mv /path/to/file_location /path/to/new/file_location
# Note, if you manually move a file or rename it (w/o git mv) git may
# treat the moved/renamed file as a new file
# Note, it's fairly popular to use the alias gm for git mv