ancestor.deepest: decrement ninteresting correctly (issue3984)

The invariant this code tries to hold is that ninteresting is the number of
non-zero elements in the interesting array. interesting[nsp] is incremented at
the same time as interesting[sp] is decremented. So if interesting[nsp] was
previously 0, ninteresting shouldn't be decremented.

mercurial/parsers.c

@@ -1362,10 +1362,10 b' static PyObject *find_deepest(indexObjec'
 				if (nsp == sp)
 					continue;
 				seen[p] = nsp;
+				interesting[sp] -= 1;
+				if (interesting[sp] == 0 && interesting[nsp] > 0)
+					ninteresting -= 1;
 				interesting[nsp] += 1;
-				interesting[sp] -= 1;
-				if (interesting[sp] == 0)
-					ninteresting -= 1;
 			}
 		}
 		interesting[sv] -= 1;