Minor enhancements to rundiff script.
Minor enhancements to rundiff script.

util/rundiff:
    Make sure Algorithm::Diff is loaded only if you need it
    (so it's ok if you don't have it installed).
    Add command-line options for things you might want to change:
    - diff algorithm
    - # of context lines
    - # of lines of lookahead

andrewfu0325/gem5-aladdin,haowu4682/gem5,andrewfu0325/gem5-aladdin,haowu4682/gem5,haowu4682/gem5,andrewfu0325/gem5-aladdin,LingxiaoJIA/gem5,LingxiaoJIA/gem5,andrewfu0325/gem5-aladdin,LingxiaoJIA/gem5,haowu4682/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,andrewfu0325/gem5-aladdin,haowu4682/gem5,haowu4682/gem5,haowu4682/gem5,haowu4682/gem5,LingxiaoJIA/gem5,andrewfu0325/gem5-aladdin,haowu4682/gem5,andrewfu0325/gem5-aladdin