Use git mergetool to run one of several merge utilities to resolve
merge conflicts. It is typically run after git merge.
If one or more <file> parameters are given, the merge tool program will
be run to resolve differences in each file (skipping those without
conflicts). Specifying a directory will include all unresolved files in
that path. If no <file> names are specified, git mergetool will run
the merge tool program on every file with merge conflicts.