mergetools: simplify how we handle "vim" and "defaults"

Remove the exceptions for "vim" and "defaults" in the mergetool library
so that every filename in mergetools/ matches 1:1 with the name of a
valid built-in tool.

Define the trivial fallback definition of shell functions in-line in
git-mergetool-lib script, instead of dot-sourcing them from another
file.  The result is much easier to follow.

[jc: squashed in an update from John Keeping as well]

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

mergetools/vimdiff

@@ -0,0 +1,36 @@
+diff_cmd () {
+	"$merge_tool_path" -R -f -d \
+		-c 'wincmd l' -c 'cd $GIT_PREFIX' "$LOCAL" "$REMOTE"
+}
+
+merge_cmd () {
+	touch "$BACKUP"
+	case "$1" in
+	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"
+		;;
+	esac
+	check_unchanged
+}
+
+translate_merge_tool_path() {
+	case "$1" in
+	gvimdiff|gvimdiff2)
+		echo gvim
+		;;
+	vimdiff|vimdiff2)
+		echo vim
+		;;
+	esac
+}