Use monospace font when editing commit message

Layout matters on a commit message, because tools don't line wrap
automatically. Use a monospaced font like we do at display time so
the lines in the editor line up correctly.

Update the width to be 72 characters in the monospaced font, to
encourage authors to keep the width of a message within the confines
of an 80 column terminal after git log adds its formatting.

Change-Id: I8563e666602316a73b0f43eb37e353791e04fbb2