gitk: l10n: make PO headers identify the Gitk project
Commit f697d08 (gitk: i18n: use "Gitk" as package name in POT file, 2026-03-19) updated the generated POT template to use "Gitk" in its Project-Id-Version header. Several existing PO files still carry older header values such as "git" or "git-gui", so they do not consistently identify themselves as Gitk translations. Update the Project-Id-Version field in all Gitk PO files so that they identify the Gitk project consistently. The "Project-Id-Version" field in the PO header helps tools identify which project a PO file belongs to. For example, Git's "git-po-helper" uses it to choose project-specific checks and POT handling rules. Without this change, some Gitk PO files are misidentified because their headers still refer to other projects. Signed-off-by: Jiang Xin <worldhello.net@gmail.com>