We now develop in two branches, for bugfixing and features. Merging the two, if we haven't done it for a while (and we now do it every day), is a bit scary. The best tool I've found is p4merge, which does a three-way diff showing me changes in Dev, changes in Master, and the common base. A fourth pane lets me edit the output.
It works seamlessly with git - see this blog post about integrating p4merge with git mergetool. If it doesn't work, you have an old version of git - upgrade.