Merge branch 'js/fix-open-exec'

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>