with GIT:
when diff'ing between master and branch-n, how can I see a graphical view of differences?
i.e. I want to see a list of all files that are different, and a graphical view that let's me manually merge files if I want to (if I don't want to auto-merge).
Say after I manually view each file that was changed, I want to merge all, can I do that in a single command?
gitk is an excellent tool but it requires X-Server. So, if you are in Mac you need to launch X11 before running gitk. In windows you can install cygwin with X11 and of course in Linux (with x) it just works.
Try "git difftool" instead of "git diff". There are several options for the visual differencer that will be used.
You can configure the difftool of your choice. I know a couple devs who have wired up p4merge for both mergetool and difftool. Check out the git config man page for particulars.
Also, if you are on OS X, gitx is quite nice for visually display of commit history and changes made.