use get_merge_tool_path() also in is_available() to honor settings

fix the is_available test used in git mergetool --tool-help or
git difftool --tool-help or to check the list of tools available when no
tool is configured/given with --tool

symtoms: the actual tool running run_merge_tool () considers the difftool.
"$merge_tool".path and mergetool."$merge_tool".path and if configured
honors these. See get_merge_tool_path () in git-mergetool--lib.sh
If not set use fallback: translate_merge_tool_path "$merge_tool".

The is_available () just uses translate_merge_tool_path "$merge_tool".

repo 1: Configure an invalid path in mergetool."$merge_tool".path for a
tool of your choice.
You will be informed that the tool is available, but when trying to use
it will not be found because the invalid configured path is used.

repo2: Install a tool of your choice on a nonstandard place (e.g. rename
the program) and configure mergetool."$merge_tool".path for this tool.
You will be informed that the tool is not available (because not found on
standard place), but when trying to run it will work.

This fix will make the information consistent by using get_merge_tool_path()
also in is_available()

Signed-off-by: Michael Schindler <michael@compressconsult.com>

git-mergetool--lib.sh

@@ -18,7 +18,7 @@ mode_ok () {
 }
 
 is_available () {
-	merge_tool_path=$(translate_merge_tool_path "$1") &&
+	merge_tool_path=$(get_merge_tool_path "$1") &&
 	type "$merge_tool_path" >/dev/null 2>&1
 }