bug#37395: Diff-mode doesn't take into account patch-separators as produced by git-format-patch

[Konstantin Kharlamov]

As title says. As a follow-up to this report I'm gonna send a fix to 
this problem.

Observable result of the problem is that Emacs thinks `-- ` patch line 
is a "deleted line", whereas it actually is end of the patch.

The follow-up patch I successfully used to edit patch-series of 12 
patches to libinput 
https://gitlab.freedesktop.org/libinput/libinput/merge_requests/288#note_223871

# Steps to reproduce

1. In terminal, go to Emacs git repository
2. Execute `git format-patch -1 --stdout > 1.patch`
3. Open 1.patch in Emacs (diff-mode should automatically get enabled)
4. Replace at the beginning of a "deleted line" the `-` with space ` `.

## Expected

The 4-digit header (which looks like `@@ -561,7 +569,8`) should have 
first 2 digits (561 and 7 in example) unchanged.

## Actual

The 4-digit header increases count of 2nd digit (7 in example) by one.