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 test
cd test
echo foo > foo
hg add foo
hg commit -d '0 0' -m commit-1
echo foo >> foo
hg commit -d '1 4444444' -m commit-3
hg commit -d '1 15.1' -m commit-4
hg commit -d 'foo bar' -m commit-5
hg commit -d ' 1 4444' -m commit-6
hg commit -d '111111111111 0' -m commit-7
echo bar > bar
hg add bar
rm bar
hg commit -d "1000000 0" -m commit-8 2>&1 | sed -e "s:/.*\(/test/.*\):...\1:"
exit 0