feat(app): parse issue, pull request and run references

resolveRef accepts N, #N, [HOST/]OWNER/REPO#N and issue, pull request or run
URLs; a URL or qualified number selects its repository for the invocation and
must agree with --repo. resolveRefs does the same for several references.

pullSelector also accepts a head branch or OWNER:BRANCH and, without an
argument, finds the pull request of the checked-out branch: the one pr
checkout recorded in branch.<name>.tea-pr, else the open pull request whose
head is the branch's upstream in the upstream remote's repository, else one
whose head has the local branch's name, else N of a pr/N branch. This lets pr
commands find the pull request of a branch pr checkout created.

target.expandMe resolves @me in user-valued flags to the login tea acts as,
and git.CurrentBranchConfig reads the branch configuration involved.

Signed-off-by: bircni <bircni@icloud.com>

internal/git/branch.go

@@ -0,0 +1,60 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package git
+
+import (
+	"context"
+	"errors"
+	"strconv"
+	"strings"
+)
+
+// ErrNotOnBranch reports a detached HEAD or a directory outside a checkout.
+var ErrNotOnBranch = errors.New("not on a branch")
+
+// BranchConfig is the configuration of a local branch that locates its pull request.
+type BranchConfig struct {
+	// Name is the local branch.
+	Name string
+	// Remote and Merge are the remote and branch the branch tracks, Merge without refs/heads/;
+	// both are empty for a branch that tracks nothing or another local branch.
+	Remote, Merge string
+	// PullNumber and BaseRemote are what pr checkout recorded: the pull request the branch was
+	// checked out from and the remote of that pull request's base repository.
+	PullNumber int64
+	BaseRemote string
+}
+
+// CurrentBranchConfig reads the configuration of the checked-out branch, failing with
+// ErrNotOnBranch when HEAD is detached or dir is not a checkout.
+func CurrentBranchConfig(ctx context.Context, runner Runner, dir string) (BranchConfig, error) {
+	name, err := runner.Output(ctx, dir, false, "symbolic-ref", "--short", "HEAD")
+	if err != nil || name == "" {
+		return BranchConfig{}, ErrNotOnBranch
+	}
+	return ReadBranchConfig(ctx, runner, dir, name), nil
+}
+
+// ReadBranchConfig reads branch.<name>.remote, .merge and the keys pr checkout records.
+// Missing or malformed keys stay empty.
+func ReadBranchConfig(ctx context.Context, runner Runner, dir, name string) BranchConfig {
+	get := func(key string) string {
+		value, err := runner.Output(ctx, dir, false, "config", "--get", "branch."+name+"."+key)
+		if err != nil {
+			return ""
+		}
+		return value
+	}
+	config := BranchConfig{Name: name, Remote: get("remote"), BaseRemote: get(pullBaseRemoteKey)}
+	if config.Remote == "." {
+		config.Remote = ""
+	}
+	if config.Remote != "" {
+		config.Merge = strings.TrimPrefix(get("merge"), "refs/heads/")
+	}
+	if n, err := strconv.ParseInt(get(pullNumberKey), 10, 64); err == nil && n > 0 {
+		config.PullNumber = n
+	}
+	return config
+}