git-svn: correctly track revisions made to deleted branches

git-svn: correctly track revisions made to deleted branches

git-svn has never been able to handle deleted branches very well
because svn_ra_get_log() is all-or-nothing, meaning that if the
max revision passed to it does not contain the path we're
tracking, we miss all the revisions in the repository.

Branches fetched using --follow-parent still do this
sub-optimally (will be fixed soon).  --follow-parent will soon
become the default, so we will assume that when using get_log();

We will also avoid tracking revprops for revisions with no
path-related changes since otherwise we just end up pulling
logs to paths we don't care about.

Also added a test for this to t9104-git-svn-follow-parent.sh and
correctly commit the log message in the preceeding test (which
conflicted with a filename).

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