Fix vtest reporting when build dir is just _build/, not _build/sub/

See "cope with older automake" a couple of lines up

Should fix the issue seen with Solaris vtesters. bash -x output:

+ vtc=./varnish-trunk/_build/bin/varnishtest/tests/r02686.vtc
++ echo ./varnish-trunk/_build/bin/varnishtest/tests/r02686.trs
++ sed -e 's/trs$/log/'
+ logfile=./varnish-trunk/_build/bin/varnishtest/tests/r02686.log
+ log=r02686.log
++ git log -n 1 --pretty=format:%H ./varnish-trunk/_build/bin/varnishtest/tests/r02686.vtc
fatal: ambiguous argument './varnish-trunk/_build/bin/varnishtest/tests/r02686.vtc': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'
+ rev=

tools/vtest.sh

@@ -194,7 +194,7 @@ failedtests () (
 	while read trs
 	do
 		name=`basename "${trs}" .trs`
-		vtc=`echo $trs | sed -e 's/trs$/vtc/' -e 's/.*sub\///' `
+		vtc=`echo $trs | sed -e 's/trs$/vtc/' -e 's/.*_build\/\(sub\/\)\?//'`
 		logfile=`echo $trs | sed -e 's/trs$/log/'`
 		log="${name}.log"
 		rev=`git log -n 1 --pretty=format:%H "${vtc}"`