svn: use correct variable name for short OID

svn: use correct variable name for short OID

The commit 9ab33150a0 ("perl: create and switch variables for hash
constants", 2020-06-22) converted each instance of the variable
$sha1_short into $oid_short in the Subversion code, since git-svn now
understands SHA-256.  However, one conversion was missed.

As a result, Perl complains about the use of this variable:

  Use of uninitialized value $sha1_short in regexp compilation at
  /usr/lib64/perl5/vendor_perl/5.30.3/Git/SVN/Log.pm line 301, <$fh>
  line 6.

Because we're parsing raw diff output here, the likelihood is very low
that we'll actually misparse the data, since the only lines we're going
to get starting with colons are the ones we're expecting.  Even if we
had a newline in a path, we'd end up with a quoted path.  Our regex is
just less strict than we'd like it to be.

However, it's obviously undesirable that our code is emitting Perl
warnings, so let's convert it to use the proper variable name.

Reported-by: Nikos Chantziaras <realnc@gmail.com>
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>