Quote the line a comment applies to when sending comments by email

Gerrit 1.x and Reitveld included the quoted line from the source
file when sending out line level comments by email. For example,
they showed the following if the comment was placed on a line
declaring "int foo = 4":

  Line 4: int foo = 4;
  No foo. Textbooks ain't no good for learn'n.

We now (finally) do the same in Gerrit2.

Bug: GERRIT-33
Signed-off-by: Shawn O. Pearce <sop@google.com>