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

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

Documentation/config.txt

@@ -684,6 +684,7 @@ branch.<name>.rebase::
 	When true, rebase the branch <name> on top of the fetched branch,
 	instead of merging the default branch from the default remote when
 	"git pull" is run.
+	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]
 	for details).