workaround Xapian OFD locks w/o close-on-exec

workaround Xapian OFD locks w/o close-on-exec

Xapian v1.2.21..v1.2.24 (inclusive) use OFD locks but failed to
set the close-on-exec flag on those locks.  So we must continue
to work around those old versions by ensuring Xapian file
descriptors aren't held any longer than necessary when in
long-running git processes.

Reported-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>