Merge branch 'jc/cutoff-config'

Merge branch 'jc/cutoff-config'

"[gc] rerereResolved = 5.days" used to be invalid, as the variable
is defined to take an integer counting the number of days.  It now
is allowed.

* jc/cutoff-config:
  rerere: allow approxidate in gc.rerereResolved/gc.rerereUnresolved
  rerere: represent time duration in timestamp_t internally
  t4200: parameterize "rerere gc" custom expiry test
  t4200: gather "rerere gc" together
  t4200: make "rerere gc" test more robust
  t4200: give us a clean slate after "rerere gc" tests