This question shows how to set meld as the git mergetool, and notes
If you have more than 1 file with merge conflicts then git will open a new Meld window for each, one after another until they are all done.
Setting up and using Meld as your git difftool and mergetool
When I use meld with mercurial merge, I get the entire set of conflicts, one tab per file, similar to this meld command:
meld --auto-compare -output=merged-changes their-changes merged-changes my-changes
How can I configure git to achieve the same results (one meld invocation, with each conflicting file in its own tab)?