What's the difference between vimdiff and vimdiff2 in git?

Question

What's the difference between them? My search engine results talk only about vimdiff, yet the command

git mergetool 

offers me both.

Answer 1

vimdiff2 was introduced in commit 0008669 (Sept 2010, for git 1.7.4)

It is like vimdiff, but with different merge options (as commented in commit b2a6b71, git 1.8.2: "vimdiff and vimdiff2 differ only by their merge command").

It (vimdiff2) forces a 2-way merge, versus vimdiff which will use a 3-way merge if the base (common ancestor) is detected:

gvimdiff|vimdiff)
    if $base_present
    then
        "$merge_tool_path" -f -d -c 'wincmd J' \
            "$MERGED" "$LOCAL" "$BASE" "$REMOTE"
    else
        "$merge_tool_path" -f -d -c 'wincmd l' \
            "$LOCAL" "$MERGED" "$REMOTE"
    fi
    ;;
gvimdiff2|vimdiff2)
    "$merge_tool_path" -f -d -c 'wincmd l' \
        "$LOCAL" "$MERGED" "$REMOTE"
    ;;

Note that commit 7c147b7 (April 2014, for Git 2.1.0 August 2014) actually introduces vimdiff3 as well:

It's similar to the default, except that the other windows are hidden.
This ensures that removed/added colors are still visible on the main merge window, but the other windows not visible.

Specially useful with merge.conflictstyle=diff3.