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
echo "[extensions]" >> $HGRCPATH
echo "fetch=" >> $HGRCPATH
hg init a
echo a > a/a
hg --cwd a commit -d '1 0' -Ama
hg clone a b
hg clone a c
echo b > a/b
hg --cwd a commit -d '2 0' -Amb
hg --cwd a parents -q
echo % should pull one change
hg --cwd b fetch ../a
hg --cwd b parents -q
echo c > c/c
hg --cwd c commit -d '3 0' -Amc
hg --cwd c fetch -d '4 0' -m 'automated merge' ../a
ls c