…sons

Add a new configuration variable `status.compareBranches` that allows
users to specify a space-separated list of branches to compare against
the current branch in `git status` output.

Each branch in the list can be:
- A remote-tracking branch name (e.g., `origin/main`)
- The special reference `@{upstream}` for the tracking branch
- The special reference `@{push}` for the push destination

When not configured, the default behavior is equivalent to setting
`status.compareBranches = @{upstream}`, preserving backward compatibility.

The advice messages shown are context-aware:
- "git pull" advice is shown only when comparing against @{upstream}
- "git push" advice is shown only when comparing against @{push}
- Divergence advice is shown for upstream branch comparisons

This is useful for triangular workflows where the upstream tracking
branch differs from the push destination, allowing users to see their
status relative to both branches at once.

Example configuration:
    [status]
        compareBranches = @{upstream} @{push}

Signed-off-by: Harald Nordgren <[email protected]>

We have an error_buf() helper that functions a bit like our error()
helper, but returns NULL instead of -1. Its return type is "const char
*", but this is overly restrictive. If we use the helper in a function
that returns non-const "char *", the compiler will complain about
the implicit cast from const to non-const.

Meanwhile, the const in the helper is doing nothing useful, as it only
ever returns NULL. Let's drop the const, which will let us use it in
both types of function.

Signed-off-by: Jeff King <[email protected]>