Support GIT_EDITOR='vim -c "set textwidth=80"'

If there are shell special characters in the editor string, we must
use /bin/sh to parse and execute it, rather than trying to rely on
a simple split(' ').  This avoids vim starting up with two empty
buffers, due to a misparsed command line.

Signed-off-by: Shawn O. Pearce <sop@google.com>