sha1_name: refactor upstream_mark

We will be adding new mark types in the future, so separate
the suffix data from the logic.
branch_get: return early on error

Right now we simply check if "ret" is valid before doing
further processing. As we add more processing, this will
become more and more cumbersome. Instead, let's just check
whether "ret" is invalid and return early with the error.
branch_get: provide per-branch pushremote pointers

When a caller uses branch_get to retrieve a "struct branch",
they get the per-branch remote name and a pointer to the
remote struct. However, they have no way of knowing about
the per-branch pushremote from this interface.

Let's expose that information via fields similar to
"remote" and "remote_name".

We have to do a little refactoring around the configuration
reading here. Instead of pushremote_name being its own
allocated string, it instead becomes a pointer to one of:

  1. The pushremote_name of the current branch, if
     configured.

  2. The globally configured remote.pushdefault, which we
     store separately as pushremote_config_default.

We can then set the branch's "pushremote" field by doing the
normal sequence of config fallback.
implement @{publish} shorthand

In a triangular workflow, you may have a distinct
@{upstream} that you pull changes from, but publish by
default (if you typed "git push") to a different remote (or
a different branch on the remote). It may sometimes be
useful to be able to quickly refer to that publishing point
(e.g., to see which changes you have that have not yet been
published).

This patch introduces the <branch>@{publish} shorthand (or
"@{pu}" to be even shorter). It refers to the tracking
branch of the remote branch to which you would push if you
were to push the named branch. That's a mouthful to explain,
so here's an example:

  $ git checkout -b foo origin/master
  $ git config remote.pushdefault github
  $ git push