Allow insteadof to persist for manifest_version usage.

The sync'ing workflow through here is a fair bit convoluted- at the
very least for a fresh checkout the previous version of PrepForChanges
would sync against http, than force later syncs to hit gerrit directly.

With recent changes (Ic049a8), the pushinsteadof and insteadof url
manipulations were gutted due to changes in the mirror urls used-
thus this code wound up accessing http rather than ssh.

Hack it to allow it's previous insteadof overrides to persist for
the duration of it's operations; this should restore it, and will
ultimately be removed once I've had enough time to fully spelunk the
sucker and refactor it.

(cherry picked from commit 5dea008f81270bae4f2b930719cf8e6b1c1dddbc)
(original CL Icbe20fcab363af0ddda83d8a3385f296c7571d00)

BUG=chromium-os:24664, chromium-os:24709
TEST=Run manifest_version_unittest.py, examine output.

Change-Id: I57a5a25287d1af5920e27b516b95a0314fa31e22
Reviewed-on: https://gerrit.chromium.org/gerrit/13822
Tested-by: Brian Harring <ferringb@chromium.org>
Reviewed-by: David McMahon <djmm@chromium.org>
5 files changed