git checkout [branch where you want to merge]
git checkout [branch where files are located] [path_to_file1] [path_to_file2...]
//You can now see the file added to your next commit with
git status
//Dont forget to commit
git commit -m "message"
//e.g:
git checkout master
git checkout dev src/a_specific_file.c
git commit -m "merged a_specific_file in master"