Explicitly add the default "git pull" behaviour to .git/config on clone

Explicitly add the default "git pull" behaviour to .git/config on clone

Without any specification in the .git/config file, git-pull will execute
"git-pull origin"; which in turn defaults to pull from the first "pull"
definition for the remote, "origin".

This is a difficult set of defaults to track for a new user, and it's
difficult to see what tells git to do this (especially when it is
actually hard-coded behaviour).  To ameliorate this slightly, this patch
explicitly specifies the default behaviour during a clone using the
"branch" section of the config.

For example, a clone of a typical repository would create a .git/config
containing:
  [remote "origin"]
  url = proto://host/repo.git
  fetch = refs/heads/master:refs/remotes/origin/master
  [branch "master"]
  remote = origin
  merge = refs/heads/master

The [branch "master"] section is such that there is no change to the
functionality of git-pull, but that functionality is now explicitly
documented.

Signed-off-by: Andy Parkins <andyparkins@gmail.com>