git-svn: allow UUID to be manually remapped via rewriteUUID

git-svn: allow UUID to be manually remapped via rewriteUUID

In certain situations it may be necessary to manually remap an svn
repostitory UUID. For example:

                  o--- [git-svn clone]
                 /
[origin svn repo]
                 \
                  o--- [svnsync clone]

Imagine that only "git-svn clone" and "svnsync clone" are made available
to external users. Furthur, "git-svn clone" contains only trunk, and for
reasons unknown, "svnsync clone" is missing the revision properties that
normally provide the origin svn repo's UUID.

A git user who has cloned the "git-svn clone" repo now wishes to use
git-svn to pull in the missing branches from the "synsync clone" repo.
In order for git-svn to get the history correct for those branches,
it needs to know the origin svn repo's UUID. Hence rewriteUUID.

Signed-off-by: Jay Soffian <jaysoffian@gmail.com>
Acked-by: Eric Wong <normalperson@yhbt.net>