git-svn: make (multi-)fetch safer but slower

git-svn: make (multi-)fetch safer but slower

get_log with explicit paths is the safest way to get revisions
that change a particular path we're interested in.
Unfortunately that means we still have to run get_log multiple
times for each path we're interested in, and even more if
a path gets deleted.

The first argument of get_log() is an array reference, but we
shouldn't use more than one element in that array ref because
the non-existence of _one_ of those paths for a particular range
would cause an error for all paths in that range, so yes, we
need multiple get_log calls to be on the safe side...

Signed-off-by: Eric Wong <normalperson@yhbt.net>