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....

The best solution to achieve this

create .gitignore file in repository root, and if you want to include only .c file then you need to add below lines to .gitignore file

*.*
!*.c

this will include all .c file from directory and subdirectory recursively.

using

*
!*.c

will not work on all version of git.

Tested on

git version 2.12.2.windows.2