gitk: l10n: make PO headers identify the Gitk project

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>