Often people use git pull without giving any parameter.
Traditionally, this has been equivalent to saying git pull
origin. However, when configuration branch.<name>.remote is
present while on branch <name>, that value is used instead of
origin.
In order to determine what URL to use to fetch from, the value of
the configuration remote.<origin>.url is consulted and if there
is not any such variable, the value on the URL: line in
$GIT_DIR/remotes/<origin> is used.
In order to determine what remote branches to fetch (and
optionally store in the remote-tracking branches) when the
command is run without any refspec parameters on the command
line, values of the configuration variable remote.<origin>.fetch
are consulted, and if there aren't any, $GIT_DIR/remotes/<origin>
is consulted and its Pull: lines are used. In addition to the
refspec formats described in the OPTIONS section, you can have a
globbing refspec that looks like this:
refs/heads/*:refs/remotes/origin/*
A globbing refspec must have a non-empty RHS (i.e. must store
what were fetched in remote-tracking branches), and its LHS and
RHS must end with /*. The above specifies that all remote
branches are tracked using remote-tracking branches in
refs/remotes/origin/ hierarchy under the same name.
The rule to determine which remote branch to merge after fetching
is a bit involved, in order not to break backward compatibility.
If explicit refspecs were given on the command line of git pull,
they are all merged.
When no refspec was given on the command line, then git pull uses
the refspec from the configuration or $GIT_DIR/remotes/<origin>.
In such cases, the following rules apply:
1. If branch.<name>.merge configuration for the current branch
<name> exists, that is the name of the branch at the remote
site that is merged.
2. If the refspec is a globbing one, nothing is merged.
3. Otherwise the remote branch of the first refspec is merged.
