Use git difftool for merge.tool identifiers

[moguls753]

As mentioned in rails/rails#54912, Thor’s merge_tool used to invoke the raw Git merge.tool value
directly, which fails for built‑in identifiers like vimdiff3.

This change updates merge_tool to:

1. Use ENV["THOR_MERGE"] if set, or

2. Fall back to running git difftool --no-index

git difftool will respect the diff.tool configuration (and pick a suitable default if none is set).

Fixes rails/rails#54912

[Edouard-chin]

Unfortunately I don't think this works. git mergetool will only work when there are conflicting files when attempting to merge (in the context of git). In thor's case there is no "conflict", so git mergetool will just bailout.

[moguls753]

Yes you're right. Maybe it would be better to use git difftool --no-index instead, since git mergetool is looking for conflicts in git's index. The manpage of git difftool says:

git difftool falls back to git mergetool config variables when the difftool equivalents have not been defined.

If nothing is configured you'll get:

This message is displayed because 'diff.tool' is not configured.
See 'git difftool --tool-help' or 'git help config' for more details.
'git difftool' will now attempt to use one of the following tools:
opendiff kdiff3 tkdiff xxdiff meld kompare gvimdiff diffuse diffmerge ecmerge p4merge araxis bc codecompare smerge nvimdiff vimdiff emerge
Viewing (1/1): '~/app/config/application.rb20250418-22187-1vvz3o.rb'
Launch 'vimdiff' [Y/n]?

This seems like the best approach; adding --no-prompt will suppress the configuration message. What do you think?

[Edouard-chin]

You may be able to confirm but this doesn't work either. git difftool will create 2 readonly buffers. So users will be presented with the diff but won't be able to make any changes. Also I don't think --no-index is a thing on the difftool command.

I had a quick look to find a solution and this is way more tricky. Basically the problem is that running a git command is not suited for what we are trying to do.
Our best bet if we wanted to stick with git and respect a user's configuration would be to create our own shell executable and use the git mergetool-lib https://git-scm.com/docs/git-mergetool--lib

But tbh this seems like a lot of work and it's probably best to ask a maintainer of this repo whether they'd be ok with such change.

[moguls753]

Thanks for your feedback! You're right git difftool opens 2 readonly buffers, but this is just a flag, you can in fact save your changes, either by using :wq! or by unsetting the flag via :set noreadonly. I think it just calls vim -R internally. From :help readonly:

'readonly' 'ro' boolean (default off)
local to buffer |local-noglobal|
If on, writes fail unless you use a '!'. Protects you from
accidentally overwriting a file. Default on when Vim is started
in read-only mode ("vim -R") or when the executable is called "view".

Regarding --no-index: git difftool is just a frontend for git diff, it does support the --no-index flag as we need it in this context.

[Edouard-chin]

Thanks for the explanation! Great, then that make sense to me 👍 .