Is there a way to tell git to only include certain files instead of ignoring certain files?

I haven't had need to try this myself, but from my reading of TFM it looks like a negated pattern would do what you want. You can override entries in .gitignore with later negated entries. Thus you could do something like:

*.c
!frob_*.c
!custom.c

To have it ignore all .c files except custom.c and anything starting with "frob_"

create .gitignore file in your repository and you want to track only c files and ignore all other files then add the following lines to it....

*
!*.c

'*' will ignore all files

and ! will negate files be to ignored....so here we are asking git not to ignore c files....