diff --git a/contrib/devtools/github-merge.py b/contrib/devtools/github-merge.py
index f1b6a12fd..3fee39143 100755
--- a/contrib/devtools/github-merge.py
+++ b/contrib/devtools/github-merge.py
@@ -78,24 +78,53 @@ def get_symlink_files():
             ret.append(f.decode('utf-8').split("\t")[1])
     return ret
 
-def tree_sha512sum():
-    files = sorted(subprocess.check_output([GIT, 'ls-tree', '--full-tree', '-r', '--name-only', 'HEAD']).splitlines())
+def tree_sha512sum(commit='HEAD'):
+    # request metadata for entire tree, recursively
+    files = []
+    blob_by_name = {}
+    for line in subprocess.check_output([GIT, 'ls-tree', '--full-tree', '-r', commit]).splitlines():
+        name_sep = line.index(b'\t')
+        metadata = line[:name_sep].split() # perms, 'blob', blobid
+        assert(metadata[1] == b'blob')
+        name = line[name_sep+1:]
+        files.append(name)
+        blob_by_name[name] = metadata[2]
+
+    files.sort()
+    # open connection to git-cat-file in batch mode to request data for all blobs
+    # this is much faster than launching it per file
+    p = subprocess.Popen([GIT, 'cat-file', '--batch'], stdout=subprocess.PIPE, stdin=subprocess.PIPE)
     overall = hashlib.sha512()
     for f in files:
+        blob = blob_by_name[f]
+        # request blob
+        p.stdin.write(blob + b'\n')
+        p.stdin.flush()
+        # read header: blob, "blob", size
+        reply = p.stdout.readline().split()
+        assert(reply[0] == blob and reply[1] == b'blob')
+        size = int(reply[2])
+        # hash the blob data
         intern = hashlib.sha512()
-        fi = open(f, 'rb')
-        while True:
-            piece = fi.read(65536)
-            if piece:
+        ptr = 0
+        while ptr < size:
+            bs = min(65536, size - ptr)
+            piece = p.stdout.read(bs)
+            if len(piece) == bs:
                 intern.update(piece)
             else:
-                break
-        fi.close()
+                raise IOError('Premature EOF reading git cat-file output')
+            ptr += bs
         dig = intern.hexdigest()
+        assert(p.stdout.read(1) == b'\n') # ignore LF that follows blob data
+        # update overall hash with file hash
         overall.update(dig.encode("utf-8"))
         overall.update("  ".encode("utf-8"))
         overall.update(f)
         overall.update("\n".encode("utf-8"))
+    p.stdin.close()
+    if p.wait():
+        raise IOError('Non-zero return value executing git cat-file')
     return overall.hexdigest()