remote.c: drop "remote" pointer from "struct branch"

When we create each branch struct, we fill in the "remote_name" field from the config, and then fill in the actual "remote" field (with a "struct remote") based on that name. However, it turns out that nobody really cares about the latter field. The only two sites that access it at all are: 1. git-merge, which uses it to notice when the branch does not have a remote defined. But we can easily replace this with looking at remote_name instead. 2. remote.c itself, when setting up the @{upstream} merge config. But we don't need to save the "remote" in the "struct branch" for that; we can just look it up for the duration of the operation. So there is no need to have both fields; they are redundant with each other (the struct remote contains the name, or you can look up the struct from the name). It would be nice to simplify this, especially as we are going to add matching pushremote config in a future patch (and it would be nice to keep them consistent). So which one do we keep and which one do we get rid of? If we had a lot of callers accessing the struct, it would be more efficient to keep it (since you have to do a lookup to go from the name to the struct, but not vice versa). But we don't have a lot of callers; we have exactly one, so efficiency doesn't matter. We can decide this based on simplicity and readability. And the meaning of the struct value is somewhat unclear. Is it always the remote matching remote_name? If remote_name is NULL (i.e., no per-branch config), does the struct fall back to the "origin" remote, or is it also NULL? These questions will get even more tricky with pushremotes, whose fallback behavior is more complicated. So let's just store the name, which pretty clearly represents the branch.*.remote config. Any lookup or fallback behavior can then be implemented in helper functions. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>