Verify that user supplied a command for --extcmd

If the user executed "git diffall --extcmd", it would result in a shift error with no useful message to the user. The script now verifies that a command was specified after --extcmd if it is the last option on the command-line. If it fails, an error message and usage instructions are displayed. Signed-off-by: Tim Henigan <tim.henigan@gmail.com>

Author: thenigan

git-diffall

@@ -76,8 +76,14 @@ do
 copy_back=1
 ;;
 -x|--e|--ex|--ext|--extc|--extcm|--extcmd)
-diff_tool=$2
-shift
+if test -z $2
+then
+echo You must specify the tool for use with --extcmd
+usage
+else
+diff_tool=$2
+shift
+fi
 ;;
 --)
 path_sep=1