git-integration(1): fix formatting of config variables list

Definition list entries need a trailing "::" to indicate that it is a
definition list and not just a normal paragraph with strange wrapping
and indentation.  Add this to the entries in the configuration variables
list.

Signed-off-by: John Keeping <john@keeping.me.uk>
Simplify usage examples

This reduces the different types of usage to three examples and makes
the man page content and that produced by 'git integration -h' the same.

We now have:

	git integration --create <name> [<base>]
	git integration [<action>...] [<branch>]
	git integration --continue | --abort

This is an improvement on the previous version which was missing several
options and would grow unwieldy if all of the available options are
added.

The three cases given above simply identify the one case where the
branch name is required (and base branch is optional), the cases where a
branch name is optional and any number of actions may be specified and
the cases where only a single option may be provided.

Signed-off-by: John Keeping <john@keeping.me.uk>
Add parens around '--continue | --abort' in usage

This is the Git convention to indicate that one of --continue or --abort
must be specified.

Signed-off-by: John Keeping <john@keeping.me.uk>