I would like to request support for 3-way merge using external diff tool instead of the default auto-merge. The feature can be seen for example in [http://doc.bazaar.canonical.com/latest/en/user-guide/resolving_conflicts.html|bazaar docs] or [http://www.perforce.com/perforce/products/merge.html|perforce web] and was discussed [http://www.mail-archive.com/fossil-users@lists.fossil-scm.org/msg01854.html|on the mailing list].

Bazaar creates 3 files in addition to the auto-merged file, which can be passed to the external merge tool. A bzr resolve command is then called which checks the merged file and deletes the temporary files.
Perforce uses internal merge tool which accesses the repository directly.
Since the Perforce-way would require support for accessing fossil repos directly, a reasonable way would be using the temporary files as bazaar does.

Many tools are capable of this type of merge (meld, diffuse, kdiff, winmerge, ...)