diff --git a/gitrevise/odb.py b/gitrevise/odb.py
index 3dcffef..289d282 100644
--- a/gitrevise/odb.py
+++ b/gitrevise/odb.py
@@ -8,6 +8,7 @@
 import re
 import os
 from typing import (
+    Iterator,
     TypeVar,
     Type,
     Dict,
@@ -73,7 +74,9 @@ def short(self) -> str:
     def for_object(cls, tag: str, body: bytes) -> Oid:
         """Hash an object with the given type tag and body to determine its Oid"""
         hasher = hashlib.sha1()
-        hasher.update(tag.encode() + b" " + str(len(body)).encode() + b"\0" + body)
+        hasher.update(f"{tag} {len(body)}".encode())
+        hasher.update(b"\0")
+        hasher.update(body)
         return cls(hasher.digest())
 
     def __repr__(self) -> str:
@@ -298,24 +301,41 @@ def new_commit(
         """Directly create an in-memory commit object, without persisting it.
         If a commit object with these properties already exists, it will be
         returned instead."""
-        if author is None:
-            author = self.default_author
-        if committer is None:
-            committer = self.default_committer
-
-        body = b"tree " + tree.oid.hex().encode() + b"\n"
-        for parent in parents:
-            body += b"parent " + parent.oid.hex().encode() + b"\n"
-        body += b"author " + author.replace(b"\n", b"\n ") + b"\n"
-        body += b"committer " + committer.replace(b"\n", b"\n ") + b"\n"
 
-        body_tail = b"\n" + message
-        if self.sign_commits:
-            gpgsig = self.get_gpgsig(body + body_tail)
-            body += b"gpgsig " + gpgsig.replace(b"\n", b"\n ") + b"\n"
-        body += body_tail
-
-        return Commit(self, body)
+        def header_kvs(gpgsig: Optional[bytes]) -> Iterator[Tuple[bytes, bytes]]:
+            """Yields each header name and value."""
+            yield b"tree", tree.oid.hex().encode()
+            yield from ((b"parent", p.oid.hex().encode()) for p in parents)
+            yield b"author", author or self.default_author
+            yield b"committer", committer or self.default_committer
+            if gpgsig:
+                yield b"gpgsig", gpgsig
+
+        def body_parts(gpgsig: Optional[bytes]) -> Iterator[bytes]:
+            """Yields each chunk of the body for rendering into a contiguous buffer."""
+            for key, value in header_kvs(gpgsig=gpgsig):
+                # Key, space, value (with embedded newlines indented by space), newline.
+                yield from (key, b" ", value.replace(b"\n", b"\n "), b"\n")
+            yield b"\n"
+            yield message
+
+        def build(gpgsig: Optional[bytes] = None) -> bytes:
+            """Render the body, optionally including the given gpgsig header."""
+            return b"".join(body_parts(gpgsig=gpgsig))
+
+        def get_body() -> bytes:
+            # Generate the unsigned body.
+            unsigned_body = build()
+            if not self.sign_commits:
+                return unsigned_body
+
+            # Get the signature for the unsigned body.
+            gpgsig = self.get_gpgsig(unsigned_body)
+
+            # Include the signature as a header in the final body.
+            return build(gpgsig=gpgsig)
+
+        return Commit(self, body=get_body())
 
     def get_gpgsig(self, buffer: bytes) -> bytes:
         """Return the text of the signed commit object."""
@@ -358,9 +378,11 @@ def entry_key(pair: Tuple[bytes, Entry]) -> bytes:
                 return name + b"/"
             return name
 
-        body = b""
-        for name, entry in sorted(entries.items(), key=entry_key):
-            body += cast(bytes, entry.mode.value) + b" " + name + b"\0" + entry.oid
+        body = b"".join(
+            field
+            for name, entry in sorted(entries.items(), key=entry_key)
+            for field in (cast(bytes, entry.mode.value), b" ", name, b"\0", entry.oid)
+        )
         return Tree(self, body)
 
     def get_obj(self, ref: Union[Oid, str]) -> GitObj: