lei_saved_search: avoid reentrancy in ->is_dup

lei_saved_search: avoid reentrancy in ->is_dup

Use a separate git process when calling xoids_for to prevent
reentrancy in ->is_dup.  Reentrancy happens since LeiToMail will
call ->is_dup when inside callbacks when writing mail.

This fixes --dedupe=mid test failures in t/lei-q-save.t

I could only reproduce this consistently on a uniprocessor VM.
"schedtool -a 0x1 -e ..." could not reproduce the problem on
2 and 4-core systems.