Trim the commit message in recent activity on newlines (BB-6180)

If a commit message has the style

"""
this is the first line

here is a longer explanation, after a blank line
spanning several lines blah blah...
"""

the author cleanly intended the first line to be a summary and the rest a longer explanation. the "recent activity" list disregards the blank, starts reading the long explanation and truncates after a certain length.

The recent activity would have a better aspect if only the lines before the blank were retained: first truncate the message to the first \n\n, then apply the chars limit.

An example is again at https://bitbucket.org/dvarrazzo/psycogreen