Comment 1 for bug 526704

It is written in http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=530285

I don't think it's devscripts' job to be a general "helpful packages"
metapackage. We have enough dependencies (in the general sense) just in
support of the scripts we provide.

So adding a dependency for topgit falls in the same category, I am afraid.

Nonetheless, if you feel that it is still worth to add the suggests for topgit, you should open an upstream report and discuss your need with the developers. See https://wiki.ubuntu.com/Debian/Bugs for more information on opening Debian upstream reports.
If you have done so, please tell us the number of the upstream bug (or the link), so we can add a bugwatch that will inform us about its status. Thanks in advance.