Format the full body instead of separating title

Fix #2315: Gedit strips newlines when commit lacks a blank line

%B is the "raw body (unwrapped subject and body)". Since we're
displaying both, we can use that to avoid any changes to the message
content. %B was added to git in 1.7.2 (2010) in commit
1367b12ad623e28546ba40c435015d94e7fbb248, and fugitive requires 1.8.5 so
it's safe to use.

autoload/fugitive.vim

@@ -3185,7 +3185,7 @@ function! fugitive#BufReadCmd(...) abort
         if b:fugitive_display_format
           call s:ReplaceCmd([dir, 'cat-file', b:fugitive_type, rev])
         else
-          call s:ReplaceCmd([dir, '-c', 'diff.noprefix=false', '-c', 'log.showRoot=false', 'show', '--no-color', '-m', '--first-parent', '--pretty=format:tree%x20%T%nparent%x20%P%nauthor%x20%an%x20<%ae>%x20%ad%ncommitter%x20%cn%x20<%ce>%x20%cd%nencoding%x20%e%n%n%s%n%n%b', rev])
+          call s:ReplaceCmd([dir, '-c', 'diff.noprefix=false', '-c', 'log.showRoot=false', 'show', '--no-color', '-m', '--first-parent', '--pretty=format:tree%x20%T%nparent%x20%P%nauthor%x20%an%x20<%ae>%x20%ad%ncommitter%x20%cn%x20<%ce>%x20%cd%nencoding%x20%e%n%n%B', rev])
           keepjumps 1
           keepjumps call search('^parent ')
           if getline('.') ==# 'parent '