build/i18n-merge-master: ignore apps deleted in master

build/i18n-merge-master: ignore apps deleted in master

Refine the code to ignore apps that have been deleted from master
but still exist in release branches. E.g. luci-app-samba

Previously the unhandled git error from non-existing master
mangled the .po files in the release branch: the 18n header was
removed and all non-ASCII chars were deleted from translation.

Fix this by processing only those files where 'git show' succeeds.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>
(cherry picked from commit e4baee8d25 in openwrt-19.07)