Fetch: default remote repository from branch properties

If in branch "foo" and this in config:

[branch "foo"]
       remote=bar

"git fetch" = "git fetch bar"
"git  pull" = "git pull  bar"

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

Documentation/config.txt

@@ -119,6 +119,9 @@ apply.whitespace::
 	Tells `git-apply` how to handle whitespaces, in the same way
 	as the '--whitespace' option. See gitlink:git-apply[1].
 
+branch.<name>.remote::
+	When in branch <name>, it tells `git fetch` which remote to fetch.
+
 pager.color::
 	A boolean to enable/disable colored output when the pager is in
 	use (default is true).