eoie: default to not writing EOIE section

eoie: default to not writing EOIE section

Since 3b1d9e04 (eoie: add End of Index Entry (EOIE) extension,
2018-10-10) Git defaults to writing the new EOIE section when writing
out an index file.  Usually that is a good thing because it improves
threaded performance, but when a Git repository is shared with older
versions of Git, it produces a confusing warning:

  $ git status
  ignoring EOIE extension
  HEAD detached at 371ed0defa
  nothing to commit, working tree clean

Let's introduce the new index extension more gently.  First we'll roll
out the new version of Git that understands it, and then once
sufficiently many users are using such a version, we can flip the
default to writing it by default.

Introduce a '[index] recordEndOfIndexEntries' configuration variable
to allow interested users to benefit from this index extension early.

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