git-commit-graph.txt: fix list rendering

The first list item follows immediately on the paragraph where we
introduce the list. This makes the "*" render literally as part of one
huge paragraph. (With AsciiDoc, everything is fine after that, but with
Asciidoctor, we get some minor follow-on errors.) Add an empty line --
with a list continuation ("+") -- to make the first list item render ok.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: Martin Ågren

Documentation/git-commit-graph.txt

@@ -68,6 +68,7 @@ chain of multiple commit-graph files stored in
 strategy and other splitting options. The new commits not already in the
 commit-graph are added in a new "tip" file. This file is merged with the
 existing file if the following merge conditions are met:
++
 * If `--split=no-merge` is specified, a merge is never performed, and
 the remaining options are ignored. `--split=replace` overwrites the
 existing chain with a new one. A bare `--split` defers to the remaining