ieot: default to not writing IEOT section

As with EOIE, popular versions of Git do not support the new IEOT
extension yet.  When accessing a Git repository written by a more
modern version of Git, they correctly ignore the unrecognized section,
but in the process they loudly warn

	ignoring IEOT extension

resulting in confusion for users.  Introduce the index extension more
gently by not writing it yet in this first version with support for
it.  Soon, once sufficiently many users are running a modern version
of Git, we can flip the default so users benefit from this index
extension by default.

Introduce a '[index] recordOffsetTable' configuration variable to
control whether the new index extension is written.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: jrn

Documentation/config/index.txt

@@ -5,6 +5,13 @@ index.recordEndOfIndexEntries::
 	reading the index using Git versions before 2.20. Defaults to
 	'false'.
 
+index.recordOffsetTable::
+	Specifies whether the index file should include an "Index Entry
+	Offset Table" section. This reduces index load time on
+	multiprocessor machines but produces a message "ignoring IEOT
+	extension" when reading the index using Git versions before 2.20.
+	Defaults to 'false'.
+
 index.threads::
 	Specifies the number of threads to spawn when loading the index.
 	This is meant to reduce index load time on multiprocessor machines.

read-cache.c

@@ -2698,6 +2698,15 @@ static int record_eoie(void)
 	return 0;
 }
 
+static int record_ieot(void)
+{
+	int val;
+
+	if (!git_config_get_bool("index.recordoffsettable", &val))
+		return val;
+	return 0;
+}
+
 /*
  * On success, `tempfile` is closed. If it is the temporary file
  * of a `struct lock_file`, we will therefore effectively perform
@@ -2761,7 +2770,7 @@ static int do_write_index(struct index_state *istate, struct tempfile *tempfile,
 	else
 		nr_threads = 1;
 
-	if (nr_threads != 1) {
+	if (nr_threads != 1 && record_ieot()) {
 		int ieot_blocks, cpus;
 
 		/*