git/Documentation/fetch-options.txt

-a, \--append::
	Append ref names and object names of fetched refs to the
	existing contents of `.git/FETCH_HEAD`.  Without this
	option old data in `.git/FETCH_HEAD` will be overwritten.

-f, \--force::
	When `git-fetch` is used with `<rbranch>:<lbranch>`
	refspec, it refuses to update the local branch
	`<lbranch>` unless the remote branch `<rbranch>` it
	fetches is a descendant of `<lbranch>`.  This option
	overrides that check.

-t, \--tags::
	By default, the git core utilities will not fetch and store
	tags under the same name as the remote repository;  ask it
	to do so using `--tags`.  Using this option will bound the
	list of objects pulled to the remote tags.  Commits in branches
	beyond the tags will be ignored.

-u, \--update-head-ok::
	By default `git-fetch` refuses to update the head which
	corresponds to the current branch.  This flag disables the
	check.  Note that fetching into the current branch will not
	update the index and working directory, so use it with care.