clar: pass a string for a %s format placeholder

When `clar_parse_args()` encounters an unhandled command-line option, it
wants to print out an error message that says so, and show the option
that it encountered. However, the format claims that it wants to show a
string, whereas using `argument[1]` provided only the first character.

This was side-stepped in upstream `clar` in 7eaea8b2da (clar: argument
parsing improvements, 2025-01-16) by not even showing the option
anymore.

Let's just fix this in Git until the next time Git synchronizes with an
upstream `clar` version again.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

t/unit-tests/clar/clar.c

@@ -558,7 +558,7 @@ clar_parse_args(int argc, char **argv)
 
 		default:
 			clar_abort("Unexpected commandline argument '%s'.\n",
-				   argument[1]);
+				   argument);
 		}
 	}
 }