pull: add merge-ff-only option

pull: add merge-ff-only option

It is very typical for Git newcomers to inadvertently create merges and
worst; inadvertently pushing them. This is one of the reasons many
experienced users prefer to avoid 'git pull', and recommend newcomers to
avoid it as well.

To avoid these problems and keep 'git pull' useful, it has been
suggested that 'git pull' barfs by default if the merge is
non-fast-forward, which unfortunately would break backwards
compatibility.

This patch leaves everything in place to enable this new mode, but it
only gets enabled if the user specifically configures it; pull.mode =
merge-ff-only.

Later on this mode can be enabled by default.

For the full discussion you can read:

http://thread.gmane.org/gmane.comp.version-control.git/225146/focus=225305

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>