chg: detect chg started by chg Sometimes people may create a symbol link from hg to chg, or write a wrapper script named hg calling chg. Without $HG and $CHGHG set, this will lead to chg executes itself causing deadlock. The user will notice chg hangs for some time and aborts with a timed out message, without knowing the root cause and how to solve it. This patch sets a dummy environment variable before executing hg to detect this situation, and print a fatal message with some possible solutions. CHGINTERNALMARK is set by chg client to detect the situation that chg is started by chg. It is temporary and should be dropped to avoid possible side effects.