feat: add update mode to speed up update

Update can be slow, especially when the repository has a lot of (recursive)
submodules. To speed up update, submodules status can be lighter.

By setting update mode to normal, git status performs the default behavior.
By setting update mode to fast, git status will use the option
`--ignore-submodules=dirty`. This option will only search for commit changes
in the submodules. All modified and untracked changes will be ignored.

It is possible to control this option by two ways:
 - globally, by setting the vim option `g:magit_update_mode`
 - at repository level, by setting the git option:
   `git config vimagit.update-mode fast`
 - for the record, it is also possible to set the option globally with git:
   `git config --global vimagit.update-mode fast`

g:magit_update_mode
 * Can be set to 'normal' and 'fast'.
 * Default value is 'normal'.