Uncomment the strip variable in hgrc file
#If you want to use a shorter repo-name, you can strip off components at the beginning.
#For example to strip off /usr/local/hg/repos from /usr/local/hg/repos/code use
strip = 3
And use a template like this to edit the email subject:
template = Subject: {webroot|strip}\n
details: {webroot}/rev/{node|short}
branches: {branches}
changeset: {rev}:{node|short}
user: {author}
date: {date|date}
description: {desc}\n