ota: increase re-use during updates

- use the latest stable release tag date for all files modification time
- tweak tar command tar to ignore read-errors (so missing files don't
  result in a failure to create the installation archive)

Makefile

@@ -130,6 +130,7 @@ release_excludes = $(strip $(UPDATE_PATH_EXCLUDES:%='-x!$1%') $(UPDATE_GLOBAL_EX
 define mkupdate
 cd $(INSTALL_DIR) &&
 '$(abspath tools/mkrelease.sh)'
+--epoch="$$(git log -1 --format='%cs' "$$(git describe --tags | cut -d- -f1)")"
 $(if $(PARALLEL_JOBS),--jobs $(PARALLEL_JOBS))
 --manifest=$(or $2,koreader)/ota/package.index
 $(foreach a,$1,'$(if $(filter --%,$a),$a,$(abspath $a))') $(or $2,koreader)

frontend/ui/otamanager.lua

@@ -307,8 +307,9 @@ function OTAManager:_buildLocalPackage()
     local tar_cmd = {
         './tar',
         '--create', '--file='..self.installed_package,
+        '--mtime', tostring(Version:getBuildDate()),
         '--numeric-owner', '--owner=0', '--group=0',
-        '--no-recursion', '-C', '..',
+        '--ignore-failed-read', '--no-recursion', '-C', '..',
         '--verbatim-files-from', '--files-from', self.package_indexfile,
     }