branch doc: sprinkle a few commas for readability

branch doc: sprinkle a few commas for readability

The "--force" option can also be used when the named branch does not
yet exist, and the point of the option is the user can (re)point the
branch to the named commit even if it does.  Add 'even' before 'if'
to clarify.  Also, insert another comma after "Without -f" before
"the command refuses..." to make the text easier to parse.

Incidentally, this change should help certain versions of
docbook-xsl-stylesheets that render the original without any
whitespace between "-f" and "git".

Noticed-by: Lars Schneider <larsxschneider@gmail.com>
Helped-by: Jeff King <peff@peff.net>
Helped-by: Andreas Schwab <schwab@suse.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>