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 --debug init
echo this is a1 > a
hg add a
hg commit -m0 -d "1000000 0"
echo this is b1 > b
hg add b
hg commit -m1 -d "1000000 0"
hg manifest 1
echo this is c1 > c
hg rawcommit -p 1 -d "1000000 0" -m2 c
hg manifest 2
hg -v parents
rm b
hg rawcommit -p 2 -d "1000000 0" -m3 b
hg manifest 3
hg -v parents
echo this is a22 > a
hg rawcommit -p 3 -d "1000000 0" -m4 a
hg manifest 4
hg -v parents
echo this is c22 > c
hg rawcommit -p 1 -d "1000000 0" -m5 c
hg manifest 5
hg -v parents
# merge, but no files changed
hg rawcommit -p 4 -p 5 -d "1000000 0" -m6
hg manifest 6
hg -v parents
# no changes what-so-ever
hg rawcommit -p 6 -d "1000000 0" -m7
hg manifest 7
hg -v parents