filter-branch: remove multi-line headers in msg filter

filter-branch: remove multi-line headers in msg filter

df062010 (filter-branch: avoid passing commit message through sed)
introduced a regression when filtering commits with multi-line headers,
if the header contains a blank line.  An example of this is a gpg-signed
commit:

  $ git cat-file commit signed-commit
  tree 3d4038e029712da9fc59a72afbfcc90418451630
  parent 110eac945dc1713b27bdf49e74e5805db66971f0
  author A U Thor <author@example.com> 1112912413 -0700
  committer C O Mitter <committer@example.com> 1112912413 -0700
  gpgsig -----BEGIN PGP SIGNATURE-----
   Version: GnuPG v1

   iEYEABECAAYFAlYXADwACgkQE7b1Hs3eQw23CACgldB/InRyDgQwyiFyMMm3zFpj
   pUsAnA+f3aMUsd9mNroloSmlOgL6jIMO
   =0Hgm
   -----END PGP SIGNATURE-----

  Adding gpg

As a consequence, "filter-branch --msg-filter cat" (which should leave the
commit message unchanged) spills the signature (after the internal blank
line) into the original commit message.

The reason is that although the signature is indented, making the line a
whitespace only line, the "read" call is splitting the line based on
the shell's IFS, which defaults to <space><tab><newline>.  The leading
space is consumed and $header_line is empty, causing the "skip header
lines" loop to exit.

The rest of the commit object is then re-used as the rewritten commit
message, causing the new message to include the signature of the
original commit.

Set IFS to an empty string for the "read" call, thus disabling the word
splitting, which causes $header_line to be set to the non-empty value ' '.
This allows the loop to fully consume the header lines before
emitting the original, intact commit message.

[jc: this is literally based on MJG's suggestion]

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: James McCoy <vega.james@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>