Correct optimization from 3464f5e77f34; add a test. If there are two (or more) heads that point to the same .hgtags node, we can safely skip parsing the file in all but the last head. (In 3464f5e77f34, we were parsing the file in the first head and skipping all the others.)

#!/bin/sh
hg init
echo This is file a1 > a
hg add a
hg commit -m "commit #0" -d "1000000 0"
ls
echo This is file b1 > b
hg add b
hg commit -m "commit #1" -d "1000000 0"
hg co 0
# B should disappear
ls