Fix tree hashing with nested empty directories

When testing for directories to exclude from hashing, we must exclude
not only empty directories, but also directories that themselves contain
nothing but empty directories; in essence, we suppress adding
directories that have no files contained within their entire subtree.

While fixing this, it seemed prudent to eliminate the `names` argument
to `tree_hash()`, especially as it was not actually used to iterate
over.

Fixes JuliaLang/julia#33979 (comment)

src/GitTools.jl

@@ -250,17 +250,30 @@ function blob_hash(path::AbstractString, HashType = SHA.SHA1_CTX)
     return SHA.digest!(ctx)
 end
 
+"""
+    contains_no_files(root::AbstractString)
+
+Helper function to determine whether a directory contains no files; e.g. it is
+either empty or contains only other directories that contain nothing but other
+directories.  This is used to exclude directories from tree hashing.
+"""
+function contains_no_files(root::AbstractString)
+    for (root, dirs, files) in walkdir(root)
+        if !isempty(files)
+            return false
+        end
+    end
+    return true
+end
+
+
 """
     tree_hash(root::AbstractString)
 
 Calculate the git tree hash of a given path.  Note that attempting to take the
 tree hash of an empty directory will throw an error.
 """
-function tree_hash(
-    root::AbstractString,
-    names::Vector{String} = readdir(root);
-    HashType = SHA.SHA1_CTX,
-)
+function tree_hash(root::AbstractString; HashType = SHA.SHA1_CTX)
     entries = Tuple{String, Vector{UInt8}, GitMode}[]
     for f in readdir(root)
         # Skip `.git` directories
@@ -271,9 +284,11 @@ function tree_hash(
         filepath = abspath(root, f)
         mode = gitmode(filepath)
         if mode == mode_dir
-            names = readdir(filepath)
-            isempty(names) && continue
-            hash = tree_hash(filepath, names)
+            # If this directory contains no files, then skip it
+            contains_no_files(filepath) && continue
+
+            # Otherwise, hash it up!
+            hash = tree_hash(filepath)
         else
             hash = blob_hash(filepath)
         end

test/artifacts.jl

@@ -126,8 +126,11 @@ end
             end
 
             # Empty directories do nothing to effect the hash, so we create one with a
-            # random name to prove that it does not get hashed into the rest.
-            mkpath(joinpath(path, Random.randstring(8)))
+            # random name to prove that it does not get hashed into the rest.  Also, it
+            # turns out that life is cxomplex enough that we need to test the nested
+            # empty directories case as well.
+            rand_dir = joinpath(path, Random.randstring(8), "inner")
+            mkpath(rand_dir)
 
             # Symlinks are not followed, even if they point to directories
             symlink("foo3", joinpath(path, "foo3_link"))
@@ -148,7 +151,7 @@ end
                 cd(path) do
                     read(`git init .`)
                     read(`git add . `)
-                    read(`git commit -m "foo"`)
+                    read(`git commit --allow-empty -m "foo"`)
                     hash = chomp(String(read(`git log -1 --pretty='%T' HEAD`)))
                     println(hash)
                 end