Help.c add --guide option

Logic, but no actions, included.

The --all commands option, if given, will display first.
The --guide option's list will then be displayed.

The common commands list is only displayed if neither option,
nor a command or guide name, is given.

Signed-off-by: Philip Oakley <philipoakley@iee.org>
Help.c: add list_common_guides_help() function

Re-use list_common_cmds_help but simply change the array name.
Candidate for future refactoring to pass a pointer to the array.

The common-guides.h list was generated with a simple variant of the
generate-cmdlist.sh and command-list.txt.

Do not list User-manual and Everday Git which not follow the naming
convention, nor gitrepository-layout which doesn't fit within the
name field size.

Signed-off-by: Philip Oakley <philipoakley@iee.org>
Help doc: Include --guide option description

Note that the ability to display an individual guide was
always possible. Include this in the update.

Also tell readers how git(1) can be accessed, especially for
Git for Windows users who do not have the 'man' command.
Likewise include a commentary on how to access this page (Catch 22).

Signed-off-by: Philip Oakley <philipoakley@iee.org>