Include some parent comment context in emails

Adds some context from whatever comment you're replying to, and also
links back to each file in which there were comments.

New format looks like this:

  http://gerrit.example.com:8080/#/c/1234/src/test_file.sh
  File src/test_file.sh:

  Line 1: echo test
  > Maybe echo "Hello World!" instead
  Done

Change-Id: I2f036b20a9d90b611d0ad4aa3051dd2750582225
1 file changed