Ok, so we recently converted from SVN to Mercurial.
We are using TortoiseHG normally.

In our one repository we have all of our projects, C++ / .NET / ASP. We have about 100 projects, all using common library projects.

So it would be quite difficult task to create multiple repo's for each project.

Now, we have the default branch, and let's say branchA.
I'm working on BranchA and adding my uber changes to it, and I change a common library, let's say an extension method

I want to commit this to branchA and default, how would I go about this?

However, I don't want all my changes from branchA to be merged into default, and I don't want all the other changes from default

Hopefully this is sufficient information!