Merge branch 'js/fix-open-exec'

This addresses CVE-2025-27613, Gitk can create and truncate a user's files: When a user clones an untrusted repository and runs gitk without additional command arguments, files for which the user has write permission can be created and truncated. The option "Support per-file encoding" must have been enabled before in Gitk's Preferences. This option is disabled by default. The same happens when "Show origin of this line" is used in the main window (regardless of whether "Support per-file encoding" is enabled or not). Signed-off-by: Johannes Sixt <j6t@kdbg.org>