"git describe" was taught to dig trees deeper to find a
<commit-ish>:<path> that refers to a given blob object.
* sb/describe-blob:
builtin/describe.c: describe a blob
builtin/describe.c: factor out describe_commit
builtin/describe.c: print debug statements earlier
builtin/describe.c: rename `oid` to avoid variable shadowing
revision.h: introduce blob/tree walking in order of the commits
list-objects.c: factor out traverse_trees_and_blobs
t6120: fix typo in test name