t0108: Avoid unnecessary fork()

Use `git rev-list --max-parents=0 HEAD` instead of `git rev-list HEAD |
tail -1` to get the root commit. This works since Git 1.7.4.2.

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>

tests/t0108-patch.sh

@@ -24,7 +24,7 @@ test_expect_success 'find `cgit` signature' '
 '
 
 test_expect_success 'find initial commit' '
-	root=$(git --git-dir="$PWD/repos/foo/.git" rev-list HEAD | tail -1)
+	root=$(git --git-dir="$PWD/repos/foo/.git" rev-list --max-parents=0 HEAD)
 '
 
 test_expect_success 'generate patch for initial commit' '