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 a > a
hg ci -d '0 0' -Ama
hg an a
echo "[ui]" >> $HGRCPATH
echo "strict=True" >> $HGRCPATH
hg an a
hg annotate a
echo % should succeed - up is an alias, not an abbreviation
hg up