perf-test/T00-new: fix backup creation.

"xargs tar cf backup.tar < $manifest" recreates the tar file with each
"batch" execed by xargs. In general this results in only a fraction of
the desired files being backed up.

performance-test/T00-new.sh

@@ -5,16 +5,16 @@ test_description='notmuch new'
 . $(dirname "$0")/perf-test-lib.sh || exit 1
 
 uncache_database
-
 time_start
 
+manifest=$(mktemp manifestXXXXXX)
+find mail -type f ! -path 'mail/.notmuch/*' | sed -n '1~4 p' > $manifest
+xargs tar uf backup.tar < $manifest
+
 for i in $(seq 2 6); do
     time_run "notmuch new #$i" 'notmuch new'
 done
 
-manifest=$(mktemp manifestXXXXXX)
-
-find mail -type f ! -path 'mail/.notmuch/*' | sed -n '1~4 p' > $manifest
 # arithmetic context is to eat extra whitespace on e.g. some BSDs
 count=$((`wc -l < $manifest`))
 
@@ -26,8 +26,6 @@ perl -nle 'rename "$_.renamed", $_' $manifest
 
 time_run "new ($count mv back)" 'notmuch new'
 
-xargs tar cf backup.tar < $manifest
-
 perl -nle 'unlink $_; unlink $_.copy' $manifest
 
 time_run "new ($count rm)" 'notmuch new'