The disambiguation logic had all the pieces necessary to only print
out those blobs that were ambiguous, but they hadn't been connected.

The initial logic was added in daba53aeaf ("sha1_name.c: add support
for disambiguating other types", 2012-07-02), and when the flags were
propagated in 8a10fea49b ("get_sha1: propagate flags to child
functions", 2016-09-26) GET_OID_BLOB wasn't added to lookup_flags.

Before this change requests for blobs were simply ignored:

    $ git rev-parse e8f2^{blob}
    error: short SHA1 e8f2 is ambiguous
    hint: The candidates are:
    hint:   e8f2650052 tag v2.17.0
    hint:   e8f21caf94 commit 2013-06-24 - bash prompt: print unique detached HEAD abbreviated object name
    hint:   e8f26250fa commit 2017-02-03 - Merge pull request #996 from jeffhostetler/jeffhostetler/register_rename_src
    hint:   e8f2bc0c06 commit 2015-05-10 - Documentation: note behavior for multiple remote.url entries
    hint:   e8f2093055 tree
    hint:   e8f25a3a50 tree
    hint:   e8f28d537c tree
    hint:   e8f2cf6ec0 tree
    hint:   e8f21d02f7 blob
    hint:   e8f21d577c blob
    hint:   e8f2867228 blob
    hint:   e8f2a35526 blob
    e8f2^{blob}
    [...]

But now we'll do the right thing and only print the blobs:

    $ git rev-parse e8f2^{blob}
    error: short SHA1 e8f2 is ambiguous
    hint: The candidates are:
    hint:   e8f21d02f7 blob
    hint:   e8f21d577c blob
    hint:   e8f2867228 blob
    hint:   e8f2a35526 blob
    e8f2^{blob}
    [...]

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
---
 sha1-name.c                         | 2 ++
 t/t1512-rev-parse-disambiguation.sh | 6 +++++-
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/sha1-name.c b/sha1-name.c
index 68d5f65362..023f9471a8 100644
--- a/sha1-name.c
+++ b/sha1-name.c

@@ -971,6 +971,8 @@ static int peel_onion(const char *name, int len, struct object_id *oid,
 		lookup_flags |= GET_OID_TAG;
 	else if (expected_type == OBJ_TREE)
 		lookup_flags |= GET_OID_TREEISH;
+	else if (expected_type == OBJ_BLOB)
+		lookup_flags |= GET_OID_BLOB;
 
 	if (get_oid_1(name, sp - name - 2, &outer, lookup_flags))
 		return -1;

diff --git a/t/t1512-rev-parse-disambiguation.sh b/t/t1512-rev-parse-disambiguation.sh
index 74e7d9c178..9ce9cc3bc3 100755
--- a/t/t1512-rev-parse-disambiguation.sh
+++ b/t/t1512-rev-parse-disambiguation.sh

@@ -337,7 +337,11 @@ test_expect_success C_LOCALE_OUTPUT 'ambiguity hints respect type' '
 	test_line_count = 7 hints &&
 	git rev-parse 000000000^{tag} >stdout &&
 	test_line_count = 1 stdout &&
-	grep -q ^0000000000f8f stdout
+	grep -q ^0000000000f8f stdout &&
+	test_must_fail git rev-parse 000000000^{blob} 2>stderr &&
+	grep ^hint: stderr >hints &&
+	# 5 blobs plus intro line &&
+	test_line_count = 6 hints
 '
 
 test_expect_success C_LOCALE_OUTPUT 'failed type-selector still shows hint' '

-- 
2.17.0.290.gded63e768a