Fixed nested submodule updates

When pushing changes to repositories that are used
as nested submodules Gerrit was crashing while trying
to find commit ID in the incorrect repository
(always in the most nested one, instead of the one below).

This change fixes that, by using different RevWalkers
for different repositories.

Bug: Issue 1491
Change-Id: I4908c6c3a2804000c6ccd72b9ea9007376928a08
1 file changed