mergetool: avoid misleading message "Resetting to default..."

If no mergetool is configured in the configuration variable
merge.tool the resetting message should not be printed.

This is fixed. The message is only printed if a tool is configured
but the entry in merge.tool is invalid.

Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

git-mergetool.sh

@@ -334,7 +334,7 @@ init_merge_tool_path() {
 
 if test -z "$merge_tool"; then
     merge_tool=`git config merge.tool`
-    if ! valid_tool "$merge_tool"; then
+    if test -n "$merge_tool" && ! valid_tool "$merge_tool"; then
 	    echo >&2 "git config option merge.tool set to unknown tool: $merge_tool"
 	    echo >&2 "Resetting to default..."
 	    unset merge_tool