Because Ediff is a very useful Emacs mode (in particular for VC) i wrote a shell script to start it from the command line.

#!/bin/bash
#
# Copyright (C) 2013-2020 Ralf Hoppe <ralf@dfcgen.de>
#
# ediff.sh
#
# Usage: ediff.sh [--client] LOCAL REMOTE [MERGED [BASE]]
#
 
function abspath()
{
    [[ $1 = /* ]] && echo "$1" || echo "$PWD/${1#./}"
}
 
 
MERGE=1 # if this is a MERGE
 
if [ $# -gt 0 ] && [ "$1" = "--client" ] ; then
    LOCAL=$(abspath "$2")
    REMOTE=$(abspath "$3")
    MERGED=$(abspath "$4")
    BASE=$(abspath "$5")
    EMACS_CMD="emacsclient --create-frame"
    # NOTE: Empty list `vc-handled-backends' is a workaround for Emacs 24
    #       bug #18788, regarding vc-git mode-line.
    ELISP_COMMON=\
"(setq rho/vc-handled-backends-backup vc-handled-backends                          \
       rho/ediff-frame (selected-frame)                                            \
       cscope-initial-directory nil                                                \
       vc-handled-backends ())                                                     \
 (defun rho/ediff-quit-hook-external ()                                            \
     (setq vc-handled-backends rho/vc-handled-backends-backup)                     \
     (remove-hook 'ediff-quit-hook 'rho/ediff-quit-hook-external)                  \
     (delete-frame rho/ediff-frame))                                               \
 (add-hook 'ediff-quit-hook 'rho/ediff-quit-hook-external t)"
 
    if [ -z "$(lsof -u $USER -a -c emacs 2>/dev/null | grep -w server)" ] ; then
        emacs --rho-minimal --no-site-file &
        sleep 3
    fi
 
    sleep 1
    shift
else
    LOCAL=$(abspath "$1")
    REMOTE=$(abspath "$2")
    MERGED=$(abspath "$3")
    BASE=$(abspath "$4")
    EMACS_CMD="emacs --rho-minimal --no-site-file"
    ELISP_COMMON=\
"(setq ediff-quit-hook 'kill-emacs \
       vc-handled-backends ())"
fi
 
if [ $# -lt 2 ]; then
    echo "Usage: $(basename $0) [--client] LOCAL REMOTE [MERGED [BASE]]"
    echo "       (see also GIT-DIFFTOOL(1) and GIT-MERGETOOL(1))"
    exit 1
fi
 
echo -e "$# files:\nLOCAL=$LOCAL\nREMOTE=$REMOTE"
 
if [ $# -eq 4 ] ; then
    echo -e "MERGED=$MERGED\nBASE=$BASE"
    $EMACS_CMD --eval \
"(progn $ELISP_COMMON
        (setq-default ediff-show-clashes-only t)
        (ediff-merge-files-with-ancestor \"${LOCAL}\" \"${REMOTE}\" \"${BASE}\" nil \"${MERGED}\"))" > /dev/null
elif [ $# -eq 2 -o "${REMOTE}" = "${MERGED}" ] ; then
    $EMACS_CMD --eval \
"(progn $ELISP_COMMON
        (ediff-files \"$LOCAL\" \"$REMOTE\"))" > /dev/null
    MERGE=0
else
    echo "MERGED=$MERGED"
    $EMACS_CMD --eval \
"(progn $ELISP_COMMON
        (setq-default ediff-show-clashes-only t)
        (ediff-merge-files \"${LOCAL}\" \"${REMOTE}\" nil \"${MERGED}\"))" > /dev/null
fi
 
if [ $MERGE -ne 0 ]; then
    EGREPHITS="$(egrep -c '^(<<<<<<<|>>>>>>>)' $MERGED)"
 
    if [ $EGREPHITS -ne 0 ]; then
        TMPFILE="$(mktemp --tmpdir $(basename $MERGED).XXXXXXXXXX)"
        cp -p "$MERGED" "$TMPFILE"
        echo "Conflicts -> see file $TMPFILE"
        exit 1
    fi
fi
 
exit 0