Use the tracked organization-owned fork as the pull request head

Look at the pr create command implementation, likely in pkg/cmd/pr/create/create.go. Understand how the --head flag is parsed and how the compare URL is built for the --web flow. The GitHub compare syntax OWNER:REPOSITORY:BRANCH needs to be integrated. Check existing tests for pr create to see how to add scenarios for organization-owned forks.

Describe the feature or problem you’d like to solve

An organization can own more than one fork that could supply a pull request branch. An organization and branch alone are therefore not always enough to identify the contributor’s intended head repository.

As a contributor working in an organization with multiple forks, I want to identify the exact head repository, so that the pull request is created from the fork I intended.

Proposed solution

Allow gh pr create --head ORGANIZATION/REPOSITORY:BRANCH to identify the exact organization-owned head repository. When --web is used, translate that value into GitHub’s repository-qualified compare syntax so the pull request form opens with the intended head repository and branch selected.

Feature: Choose a specific organization-owned fork as the pull request head

  Scenario: The selected fork belongs to another organization
    Given the base repository is "org/upstream"
    And "other-org/upstream-fork" is a fork of the base repository
    When the contributor runs `gh pr create --head other-org/upstream-fork:feature-branch`
    Then a pull request is created from "other-org/upstream-fork:feature-branch"
    And the pull request targets "org/upstream"

  Scenario: The selected fork belongs to the same organization as the base repository
    Given the base repository is "org/upstream"
    And "org/upstream-fork" is a fork of the base repository
    When the contributor runs `gh pr create --head org/upstream-fork:feature-branch`
    Then a pull request is created from "org/upstream-fork:feature-branch"
    And the pull request targets "org/upstream"

  Scenario: The organization owns multiple candidate forks
    Given the base repository is "org/upstream"
    And "org/upstream-fork" is a fork of the base repository
    And "org/experimental-upstream-fork" is a fork of the base repository
    When the contributor runs `gh pr create --head org/upstream-fork:feature-branch`
    Then a pull request is created from "org/upstream-fork:feature-branch"
    And the pull request targets "org/upstream"

  Scenario: Repository-qualified head is used with web mode
    Given the base repository is "org/upstream"
    And "other-org/upstream-fork" is a fork of the base repository
    When the contributor runs `gh pr create --head other-org/upstream-fork:feature-branch --web`
    Then GitHub’s pull request form opens for "org/upstream"
    And "other-org/upstream-fork" is selected as the head repository
    And "feature-branch" is selected as the head branch

Additional context

This is a sub-issue of #10093.

GitHub’s compare flow represents an exact fork as OWNER:REPOSITORY:BRANCH. For example, other-org/upstream-fork:feature-branch can be rendered in the compare URL as other-org:upstream-fork:feature-branch.

Supporting evidence:

• GitHub Docs: comparing commits across forks
• GitHub Docs issue documenting the repository-qualified compare syntax
• GitHub Docs change adding the syntax

The syntax is documented for GHES 3.10 and newer. All currently supported GHES versions are newer than that; the exact implementation floor for older, unsupported GHES releases is unknown.