improvements to match ranking

[kostmo]

• A match closer to the end of the log should be given more weight
  • perhaps used as a tiebreaker for equal specificity?
• A literal match should outrank a regex match of the same length
  • Example from this group post refers to this build (scroll to bottom of match list):
    • Pattern 70, a regex with specificity 5, matches the string:
      ERROR: test_tracing_multiple_methods (__main__.TestScript)
    • However, pattern 34, a literal expression with specificity 4, is also an exact match. It should be considered the "best match" for this build breakage.

CC @ezyang