git/Documentation/pull-fetch-param.txt

<repository>::
	The "remote" repository that is the source of a fetch
	or pull operation.  See the section <<URLS,GIT URLS>> below.

<refspec>::
	The canonical format of a <refspec> parameter is
	`+?<src>:<dst>`; that is, an optional plus `+`, followed
	by the source ref, followed by a colon `:`, followed by
	the destination ref.
+
The remote ref that matches <src>
is fetched, and if <dst> is not empty string, the local
ref that matches it is fast forwarded using <src>.
Again, if the optional plus `+` is used, the local ref
is updated even if it does not result in a fast forward
update.
+
[NOTE]
If the remote branch from which you want to pull is
modified in non-linear ways such as being rewound and
rebased frequently, then a pull will attempt a merge with
an older version of itself, likely conflict, and fail.
It is under these conditions that you would want to use
the `+` sign to indicate non-fast-forward updates will
be needed.  There is currently no easy way to determine
or declare that a branch will be made available in a
repository with this behavior; the pulling user simply
must know this is the expected usage pattern for a branch.
+
[NOTE]
You never do your own development on branches that appear
on the right hand side of a <refspec> colon on `Pull:` lines;
they are to be updated by `git-fetch`.  If you intend to do
development derived from a remote branch `B`, have a `Pull:`
line to track it (i.e. `Pull: B:remote-B`), and have a separate
branch `my-B` to do your development on top of it.  The latter
is created by `git branch my-B remote-B` (or its equivalent `git
checkout -b my-B remote-B`).  Run `git fetch` to keep track of
the progress of the remote side, and when you see something new
on the remote branch, merge it into your development branch with
`git pull . remote-B`, while you are on `my-B` branch.
The common `Pull: master:origin` mapping of a remote `master`
branch to a local `origin` branch, which is then merged to a
local development branch, again typically named `master`, is made
when you run `git clone` for you to follow this pattern.
+
[NOTE]
There is a difference between listing multiple <refspec>
directly on `git-pull` command line and having multiple
`Pull:` <refspec> lines for a <repository> and running
`git-pull` command without any explicit <refspec> parameters.
<refspec> listed explicitly on the command line are always
merged into the current branch after fetching.  In other words,
if you list more than one remote refs, you would be making
an Octopus.  While `git-pull` run without any explicit <refspec>
parameter takes default <refspec>s from `Pull:` lines, it
merges only the first <refspec> found into the current branch,
after fetching all the remote refs.  This is because making an
Octopus from remote refs is rarely done, while keeping track
of multiple remote heads in one-go by fetching more than one
is often useful.
+
Some short-cut notations are also supported.
+
* `tag <tag>` means the same as `refs/tags/<tag>:refs/tags/<tag>`; 
  it requests fetching everything up to the given tag.
* A parameter <ref> without a colon is equivalent to
  <ref>: when pulling/fetching, so it merges <ref> into the current
  branch without storing the remote branch anywhere locally