include line number in each change header in mail

When reading the mail from a very long review,
it is difficult, looking at a single comment that
says Line 123, to scan backward in the mail to
find the immediately preceding File line.  All the
stanzas look about the same.

This patch changes the introductory text from

Line 123:

to

filename:123:

as is commonly used for compiler error messages
and the like.  This avoids the problem of having
to scan backward to figure out which file is being
discussed.

The problem is particularly acute when you get a
batch of replies that is maybe 25 "Done." across
a few different files and you see the one substantial
reply and can't immediately match it to a file.

[GvR]

LG.

Submitted as r483 and made live on codereview.appspot.com.