doc: add a blank line around block delimiters

The documentation is using the historical mode for titles, which is a
setext-style (i.e., two-line) section title.

The issue with this mode is that starting block delimiters (e.g.,
`----`) can be confused with a section title when they are exactly the
same length as the preceding line. In the original documentation, this
is taken care of for English by the writer, but it is not the case for
translations where these delimiters are hidden. A translator can
generate a line that is exactly the same length as the following block
delimiter, which leads to this line being considered as a title.

To safeguard against this issue, add a blank line before and after
block delimiters where block is at root level, else add a "+" line
before block delimiters to link it to the preceding paragraph.

Signed-off-by: Jean-Noël Avila <jn.avila@free.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-rebase.adoc

@@ -1107,10 +1107,12 @@ In that case, the fix is easy because 'git rebase' knows to skip
 changes that are already present in the new upstream (unless
 `--reapply-cherry-picks` is given). So if you say
 (assuming you're on 'topic')
+
 ------------
     $ git rebase subsystem
 ------------
 you will end up with the fixed history
+
 ------------
     o---o---o---o---o---o---o---o  master
 				 \
@@ -1145,6 +1147,7 @@ of the old 'subsystem', for example:
 
 You can then transplant the old `subsystem..topic` to the new tip by
 saying (for the reflog case, and assuming you are on 'topic' already):
+
 ------------
     $ git rebase --onto subsystem subsystem@{1}
 ------------