Merge branch 'sb/describe-blob' into next

"git describe $garbage" stopped giving any errors when the garbage
happens to be a string with 40 hexadecimal letters.

* sb/describe-blob:
describe: confirm that blobs actually exist

diff --cc builtin/describe.c
Simple merge

diff --cc t/t6120-describe.sh
index a5d901502414f25616a474152ee0f5816465bd37,50029ff6a8df6c0e11a3f5dea75905062e088b97..bae78c4e89e2402d5ab37e3b7cd094dde41d69b3
--- 1/t/t6120-describe.sh
--- 2/t/t6120-describe.sh
+++ b/t/t6120-describe.sh
@@@ -374,8 -374,12 +374,16 @@@ test_expect_success ULIMIT_STACK_SIZE '
        test_cmp expect actual
  '
  
 +check_describe tags/A --all A
 +check_describe tags/c --all c
 +check_describe heads/branch_A --all --match='branch_*' branch_A
 +
+ test_expect_success 'describe complains about tree object' '
+       test_must_fail git describe HEAD^{tree}
+ '
+ 
+ test_expect_success 'describe complains about missing object' '
+       test_must_fail git describe $_z40
+ '
+ 
  test_done