I'm not currently able to comment on meld as don't have it installed but for kdiff3(*), the
git difftool --dir-diff command works nicely, e.g. to compare any two remote branches:
git difftool --dir-diff origin/branch1..origin/branch2
For kdiff3, this lists all the files that have changed in one pane and the differences for a selected file are shown in another pane - would imagine meld would do something similar. It can take a while before the diff comes up if there are a lot of changes.
(*) Side note - setting up kdiff: Under Windows, can install kdiff3 from here. Git can then be configured to use kdiff3 by editing
C:\Users\<your username>\.gitconfig and setting the following (assuming the path to kdiff3 below is correct and replacing existing sections if present):-
tool = kdiff3
prompt = false
cmd = "\"C:/Program Files/KDiff3/kdiff3.exe\" \"$LOCAL\" \"$REMOTE\""