commit: Fix reading xattrs from OstreeRepoFile:s

When doing commit --tree=ref=XXX while at the same time applying some
form of modifier, ostree dies trying to read the xattrs using the
raw syscalls. We fix this by falling back to ostree_repo_file_get_xattrs()
in this case.

Also adds a testcase for this.

Closes: #577
Approved by: cgwalters

tests/basic-test.sh

@@ -19,7 +19,7 @@
 
 set -euo pipefail
 
-echo "1..58"
+echo "1..59"
 
 $OSTREE checkout test2 checkout-test2
 echo "ok checkout"
@@ -178,6 +178,9 @@ echo "ok user checkout"
 $OSTREE commit -b test2 -s "Another commit" --tree=ref=test2
 echo "ok commit from ref"
 
+$OSTREE commit -b test2 -s "Another commit with modifier" --tree=ref=test2 --owner-uid=`id -u`
+echo "ok commit from ref with modifier"
+
 $OSTREE commit -b trees/test2 -s 'ref with / in it' --tree=ref=test2
 echo "ok commit ref with /"