git-config-set: Properly terminate strings with '\0'

When a lowercase version of the key was generated, it was not
terminated. Strangely enough, it worked on Linux and macosx anyway.
Just cygwin barfed.

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

config.c

@@ -432,6 +432,7 @@ int git_config_set_multivar(const char* key, const char* value,
 			return 1;
 		} else
 			store.key[i] = tolower(key[i]);
+	store.key[i] = 0;
 
 	/*
 	 * The lock_file serves a purpose in addition to locking: the new