How resolve multiple conflicts with "git mergetool" without having to close the editor between files?

At first glance, it does not seem possible to reuse an external diff tool session.

The git-mergetool documentation clearly states:

If the custom merge tool correctly indicates the success of a merge resolution with its exit code, then the configuration variable mergetool.<tool>.trustExitCode can be set to true.
Otherwise, git-mergetool will prompt the user to indicate the success of the resolution after the custom tool has exited.

So the exit code (or the validation of the user after the exit of the diff tool) is needed, implying that the user first close the external diff tool.

That seems a great incentive to reduce the number of conflicts on each merge/rebase one attempts ;) (whatever the VCScs tool used)

Note:
Two other git external diff tools settings ("Setting up diff and merge tools for Git on Windows" and "Setting up SourceGear DiffMerge with Git") do not give more hopes when it come to not closing the external diff tool...

If your mergetool of choice supports opening files in an existing instance, you can specify the command in your git config:

% git config mergetool.whatever_you_want.cmd 'exec /path/to/merge/tool $LOCAL $MERGED $REMOTE'
% git config merge.tool whatever_you_want

git mergetool will then execute your custom command and then prompt you as to whether the file was merged successfully (in lieu of looking at an exit code).

An example I just hacked together for vimdiff:

% git config mergetool.persistent.cmd 'gvim --remote-tab-silent "+set buftype=nowrite" "$PWD/$BASE" && sleep 1; gvim --remote-send ":split $PWD/$REMOTE<CR>:set buftype=nowrite<CR>:vertical diffsplit $PWD/$MERGED<CR>:vertical diffsplit $PWD/$LOCAL<CR>:set buftype=nowrite<CR><C-W>l"'

This works well enough, I may start using it myself!

The problem for mergetool is that it deliberately uses a command line interface to initiate a merge session and then waits for the invoked command to return to determine when the user driven merge has completed.

Most merge tools don't provide a command line mechanism for starting a merge session in an already running process with a way for determining when the resolution has been completed and whether successful or not.

It is conceivable that some merge tools could provide this functionality through a separate wrapper command and some sort of IPC, but it would be exceedingly tool specific and difficult to implement in the generic mergetool program.