Regression in patch add?

[mqudsi@neosmart.net]

Hello all,

I'm currently running the latest version of git built from `master`, and
I'm running into what appears to be a regression in the behavior of the
piecewise `git add -p` when applying a manually edited chunk.

I first run `git add -p`, then manually edit a chunk (after hitting `s`
once, if it matters). The chunk originally contains the following:

```diff
# Manual hunk edit mode -- see bottom for a quick guide
@@ -20,7 +20,7 @@
 	"call dein#add('Shougo/dein.vim', {'rev': 'master'})

 	" Add or remove your plugins here:
-	" call dein#add('flazz/vim-colorschemes')
-	call dein#add('Haron-Prime/evening_vim')
+	call dein#add('flazz/vim-colorschemes')
+	call dein#add('danilo-augusto/vim-afterglow')

 	"core plugins that change the behavior of vim and how we use it globally
```

Under git 2.7.4, I can edit it to the following, which is accepted
without a problem:

```diff
# Manual hunk edit mode -- see bottom for a quick guide
@@ -20,7 +20,7 @@
	"call dein#add('Shougo/dein.vim', {'rev': 'master'})

	" Add or remove your plugins here:
-	" call dein#add('flazz/vim-colorschemes')
-	call dein#add('Haron-Prime/evening_vim')
+	call dein#add('flazz/vim-colorschemes')
+	call dein#add('Haron-Prime/evening_vim')

	"core plugins that change the behavior of vim and how we use it globally
```

All I did here was remove one `+` line and manually add another (which
is a variant of the second `-` line).

Under git 2.17.0.252.gfe0a9ea, the same piece is opened in $VISUAL for
editing (and if left unmodified applies OK), but when modified in the
to the same exact value, after exiting the editor I receive the
following error from git:

    error: patch fragment without header at line 15: @@ -25,7 +25,8 @@

I'm not sure what to make of this.

Thank you,

Mahmoud Al-Qudsi
NeoSmart Technologies