Exclude subdirectories

[metaflow]

I would like to add exclusion of subdirectories to cindex. For example indexing of chromium /src takes about 50 seconds while only 20 seconds if I exclude /src/out and /src/third_party.
Right now it works like cindex --exclude /src/out --exclude /src/tmp /src plus I have to pass excludes if I want to re-index. As I understand the way to integrate that is to modify index format to store list of excluded directories. Is it OK to introduce such backward incompatible changes?