When unreachable revisions are given to "svn log", it displays all commit
logs in the given range that exist in the current tree.  (If no commit
logs are found in the current tree, it simply prints a single commit log
separator.)  This patch makes "git-svn log" behave the same way.

Ten tests added to t/t9116-git-svn-log.sh.
Signed-off-by: NDavid D Kilzer <ddkilzer@kilzer.net>
Acked-by: NEric Wong <normalperson@yhbt.net>