Re: [PATCH 2/3] ieot: default to not writing IEOT section

+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'.

Probably worth adding a test that exercises this new config option -
somehow create an index with index.recordOffsetTable=1, check that the
index contains the appropriate string (a few ways to do this, but I'm
not sure which are portable), and then run a Git command that reads the
index to make sure it is valid; then do the same except
index.recordOffsetTable=0.

The code itself looks good to me.

Same comment for patch 1.