git-svn: un-break "git svn rebase" when log.abbrevCommit=true

git-svn: un-break "git svn rebase" when log.abbrevCommit=true

Change git-svn to use git-rev-list(1) instead of git-log(1) since the
latter is porcelain that'll cause "git svn rebase" to fail completely
if log.abbrevCommit is set to true in the configuration.

Without this patch the code will fail to parse a SHA1, and then just
spew a bunch of "Use of uninitialized value $hash in string eq"
warnings at "if ($c && $c eq $hash) { ..." and never do anything
useful.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Helped-by: Jonathan Nieder <jrnieder@gmail.com>
Acked-by: Eric Wong <normalperson@yhbt.net>