View differences of branches with meld?

Short & sweet:

git config --global diff.tool meld

This configures Git to use meld as the diff tool. (You don't need to specify the command line arguments, support for meld is built into Git.)

Then, if you want a graphical diff instead of a textual one, you simply invoke git difftool instead of git diff (they both take the same arguments). In your case:

git difftool master..devel

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Update: If you don't want the one-file-at-a-time diff, but instead want to use meld's "subdirectory" view with all the changes between the two branches, note the -d or --dir-diff option for git difftool. For example, when I'm on branch XYZ and I want to see what is different between this and branch ABC, I run this:

git difftool -d ABC

Starting with git v1.7.11, you can use git difftool --dir-diff to perform a directory diff. Which works quite well with meld wihout https://github.com/wmanley/git-meld scripts.

Configure git

git config --global diff.tool meld

Use it

git difftool -d topic             // -d is --dir-diff
git difftool -d master..topic

For macOS

brew cask install meld
git config --global difftool.meld.cmd 'open -W -a Meld --args \"$LOCAL\" \"$PWD/$REMOTE\"'
git config --global difftool.meld.trustExitCode true