Document the hairy gfi_unpack_entry part of fast-import

Junio pointed out this part of fast-import wasn't very clear on
initial read, and it took some time for someone who was new to
fast-import's "dirty little tricks" to understand how this was
even working.  So a little bit of commentary in the proper place
may help future readers.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fast-import.c

@@ -1125,14 +1125,47 @@ static int store_object(
 	return 0;
 }
 
+/* All calls must be guarded by find_object() or find_mark() to
+ * ensure the 'struct object_entry' passed was written by this
+ * process instance.  We unpack the entry by the offset, avoiding
+ * the need for the corresponding .idx file.  This unpacking rule
+ * works because we only use OBJ_REF_DELTA within the packfiles
+ * created by fast-import.
+ *
+ * oe must not be NULL.  Such an oe usually comes from giving
+ * an unknown SHA-1 to find_object() or an undefined mark to
+ * find_mark().  Callers must test for this condition and use
+ * the standard read_sha1_file() when it happens.
+ *
+ * oe->pack_id must not be MAX_PACK_ID.  Such an oe is usually from
+ * find_mark(), where the mark was reloaded from an existing marks
+ * file and is referencing an object that this fast-import process
+ * instance did not write out to a packfile.  Callers must test for
+ * this condition and use read_sha1_file() instead.
+ */
 static void *gfi_unpack_entry(
 	struct object_entry *oe,
 	unsigned long *sizep)
 {
 	enum object_type type;
 	struct packed_git *p = all_packs[oe->pack_id];
 	if (p == pack_data && p->pack_size < (pack_size + 20)) {
+		/* The object is stored in the packfile we are writing to
+		 * and we have modified it since the last time we scanned
+		 * back to read a previously written object.  If an old
+		 * window covered [p->pack_size, p->pack_size + 20) its
+		 * data is stale and is not valid.  Closing all windows
+		 * and updating the packfile length ensures we can read
+		 * the newly written data.
+		 */
 		close_pack_windows(p);
+
+		/* We have to offer 20 bytes additional on the end of
+		 * the packfile as the core unpacker code assumes the
+		 * footer is present at the file end and must promise
+		 * at least 20 bytes within any window it maps.  But
+		 * we don't actually create the footer here.
+		 */
 		p->pack_size = pack_size + 20;
 	}
 	return unpack_entry(p, oe->offset, &type, sizep);