fetch: get the remote branches to merge from the branch properties

fetch: get the remote branches to merge from the branch properties

If in branch "foo" and this in config:

[branch "foo"]
      merge=bar

"git fetch": fetch from the default repository and program the "bar"
             branch to be merged with pull.

Signed-off-by: Santi Béjar <sbejar@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>