mergetool--lib: simplify guess_merge_tool()

Use a case statement instead of calling grep to find out if the editor's
name contains the string "vim".  Remove the check for emacs, as this
branch did the same as the default one anyway.
Signed-off-by: NRene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-mergetool--lib.sh

@@ -325,15 +325,14 @@ guess_merge_tool () {
 		fi
 		tools="$tools gvimdiff diffuse ecmerge araxis"
 	fi
-	if echo "${VISUAL:-$EDITOR}" | grep emacs > /dev/null 2>&1; then
-		# $EDITOR is emacs so add emerge as a candidate
-		tools="$tools emerge vimdiff"
-	elif echo "${VISUAL:-$EDITOR}" | grep vim > /dev/null 2>&1; then
-		# $EDITOR is vim so add vimdiff as a candidate
+	case "${VISUAL:-$EDITOR}" in
+	*vim*)
 		tools="$tools vimdiff emerge"
-	else
+		;;
+	*)
 		tools="$tools emerge vimdiff"
-	fi
+		;;
+	esac
 	echo >&2 "merge tool candidates: $tools"
 
 	# Loop over each candidate and stop when a valid merge tool is found.