Fix 'git' wrapper usage string

Fix 'git' wrapper usage string

8b1fa77 (Allow passing of configuration parameters in the command
line, 2010-03-26) forgot the closing ']' for the -c option.

While we're there, also rewrap.  Instead of folding the last two lines
together, try to highlight that COMMAND is required by starting a line
with it.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>