This eases the way of handling many different branches for the
superproject subscription. As the superproject subscriptions
are not yet in a release, we can still play around with them.
When a user sees "refs = refs/heads/*:refs/heads/*" they cannot tell
whether this is 1:1 matching or any match to any match without
consulting the documentation. Differentiate the matching
strategy by keys, such that we have "all = <refspec>" as well
as "matching = <refspec>".
The "all" key implies there is no interaction between the
wildcards on the left and right side, e.g.
all=refs/heads/*:refs/heads*
indicates that any branch can be subscribed to any other branch
in the given superproject.
"matching" however substitutes the wildcard on the right
with the captured value on the left.
matching=refs/heads/*:refs/heads*
allows a subscription of refs/heads/foo in the submodule to the
refs/heads/foo in the superproject.
Change-Id: I84d3a72c00f76570798880adf54ce56f974466ff
bb444175a6