Make initrepo more robust against configuration errors

When an incorrect fetch refspec was present in fwmeta, initrepo would
refuse to run, with the message:

fatal: Couldn't find remote ref refs/heads/release/8.0

Fix this by ignoring errors on the initial fetch, so that new refspecs
can be set.

Change-Id: I02245595d5fa1924b3510c1b09b78b3501dd5fe3
Always reset fetch/push refspecs to match configured value

Without this, an incorrect refspec would never be cleaned out unless
--reset was run. There doesn't seem to be any reason to preserve
existing refspecs that aren't included in the branchlist, especially now
that wildcards can be used.

Change-Id: Icfaa6fbbc00549f8643b6eac75dbb9d2e2b0ee2f