coverage.sh: conditionally copy mmdebstrap to ./shared

- only copy if local copy is newer than the copy in ./shared
 - this enables the autopkgtest to place its own version in ./shared
   which will then not be overwritten

coverage.sh

@@ -12,7 +12,10 @@ rm -f shared/cover_db.img
 # prepare image for cover_db
 guestfish -N shared/cover_db.img=disk:100M -- mkfs vfat /dev/sda
 
-cp mmdebstrap shared
+# only copy if necessary
+if [ ! -e shared/mmdebstrap ] || [ mmdebstrap -nt shared/mmdebstrap ]; then
+	cp -a mmdebstrap shared
+fi
 
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