rust-dirstatemap: correctly decrement the copies counter
This was caught when writing unit tests for the `DirstateMap`. We were always
setting `had_copy_source` to `false` since we erased the value just before.
Differential Revision: https://phab.mercurial-scm.org/D12432
{
// Enforcing
"eqeqeq" : true, // true: Require triple equals (===) for comparison
"forin" : true, // true: Require filtering for..in loops with obj.hasOwnProperty()
"freeze" : true, // true: prohibits overwriting prototypes of native objects such as Array, Date etc.
"nonbsp" : true, // true: Prohibit "non-breaking whitespace" characters.
"undef" : true, // true: Require all non-global variables to be declared (prevents global leaks)
// Environments
"browser" : true // Web Browser (window, document, etc)
}