[PATCH v4 02/29] cache.h: add a comment to object_type()

Add a comment to the object_type() function to explain what it
returns, and what the "mode" is in the "else" case.

The object_type() function dates back to 4d1012c3709 (Fix rev-list
when showing objects involving submodules, 2007-11-11). It's not
immediately obvious to someone looking at its history and how it's
come to be used.

Despite what Linus noted in 4d1012c3709 (Fix rev-list when showing
objects involving submodules, 2007-11-11) about wanting to move away
from users of object_type() relying on S_ISLNK(mode) being true here
we do currently rely on that. If this is changed to a condition to
only return OBJ_BLOB on S_ISREG(mode) then t4008, t4023 and t7415 will
have failing tests.

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
---
 cache.h | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/cache.h b/cache.h
index c2f8a8eadf6..ae0c0bef5c2 100644
--- a/cache.h
+++ b/cache.h

@@ -451,11 +451,16 @@ enum object_type {
 	OBJ_MAX
 };
 
+/*
+ * object_type() returns an object of a type that'll appear in a tree,
+ * so no OBJ_TAG is possible. This is mostly (and dates back to)
+ * consumers of the tree-walk.h API's "mode" field.
+ */
 static inline enum object_type object_type(unsigned int mode)
 {
 	return S_ISDIR(mode) ? OBJ_TREE :
 		S_ISGITLINK(mode) ? OBJ_COMMIT :
-		OBJ_BLOB;
+		OBJ_BLOB; /* S_ISREG(mode) || S_ISLNK(mode) */
 }
 
 /* Double-check local_repo_env below if you add to this list. */

-- 
2.31.0.286.gc175f2cb894