I remember in SVN, I can rename a file from foo.txt to foo2.txt and all the history will follow (log file of foo2.txt will show all history of foo.txt as well). But on Merc
Use hg log --follow foo2.txt or hg log -f foo2.txt (short form) to show you the entire history of the file before the hg rename
hg log --follow foo2.txt
hg log -f foo2.txt
hg rename