branch: add --forked <branch>

branch: add --forked <branch>

List local branches whose configured upstream
(branch.<name>.merge resolved against branch.<name>.remote)
matches any of the given <branch> arguments.

Each <branch> is interpreted against the local repository, not
against any specific remote:

  * a literal upstream short name, e.g. "origin/main" or "master"
    for a branch whose upstream is local;
  * a wildmatch pattern, e.g. "origin/*";
  * a bare configured-remote name, e.g. "origin", which resolves
    to whatever refs/remotes/origin/HEAD points at, matching how
    "git checkout -b topic origin" picks a starting point.

The literal-vs-wildcard distinction is settled at parse time so
the per-branch matching loop calls wildmatch() only for genuine
wildcards. Multiple <branch> arguments are unioned. Output is
sorted by branch name.

This is the building block for --prune-merged, which deletes the
listed branches once they have landed on their upstream.

Signed-off-by: Harald Nordgren <haraldnordgren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>