Handle the branch.<name>.rebase value 'interactive'

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Documentation/config.txt

@@ -714,6 +714,7 @@ branch.<name>.rebase::
 	instead of merging the default branch from the default remote when
 	"git pull" is run. See "pull.rebase" for doing this in a non
 	branch-specific manner.
+	When the value is `interactive`, the rebase is run in interactive mode.
 +
 *NOTE*: this is a possibly dangerous operation; do *not* use
 it unless you understand the implications (see linkgit:git-rebase[1]