bookmarks: fix head selection for merge with two bookmarked heads
A type mismatch caused the search for the other head to fail. The code is
fragile, and instead it ended up using the 'first' bookmark head, but the
ordering is undefined and it could thus randomly use the wrong bookmarkhead
and fail with:
$ hg up -q -C e@diverged
$ hg merge
abort: merging with a working directory ancestor has no effect
test --time
$ hg --time help -q help 2>&1 | grep time > /dev/null
$ hg init a
$ cd a
#if lsprof
test --profile
$ hg --profile st 2>../out
$ grep CallCount ../out > /dev/null || cat ../out
$ hg --profile --config profiling.output=../out st
$ grep CallCount ../out > /dev/null || cat ../out
$ hg --profile --config profiling.format=text st 2>../out
$ grep CallCount ../out > /dev/null || cat ../out
$ echo "[profiling]" >> $HGRCPATH
$ echo "format=kcachegrind" >> $HGRCPATH
$ hg --profile st 2>../out
$ grep 'events: Ticks' ../out > /dev/null || cat ../out
$ hg --profile --config profiling.output=../out st
$ grep 'events: Ticks' ../out > /dev/null || cat ../out
#endif
$ cd ..