You can use the grep command :
hg grep --all Foo
To address Lazy Badger concerns in comments.
$ hg init
$ echo "Dim Foo as integer" > test
$ hg commit -m "1"
$ echo "I like big butts, I cannot lie" > test
$ hg commit -m "2"
$ echo "Dim Foo as integer" > test
$ hg commit -m "3"
$ hg grep --all Foo
The output of the grep command is :
test:2:+:Dim Foo as integer
test:1:-:Dim Foo as integer
test:0:+:Dim Foo as integer
Which means, Foo was first seen in the file test on revision 0 (the + sign tells us that), then it dissapeared on revision 1 (the - signs), and reappear again on revision 2.
I don't know if it is what you want, but it clearly indicates revision on which the searched word was added or deleted.