Documentation: fix section header mark-up

Asciidoctor is stricter than AsciiDoc when deciding if underlining
is a section title or the start of preformatted text.  Make the
length of the underlining match the text to ensure that it renders
correctly in all implementations.

Signed-off-by: John Keeping <john@keeping.me.uk>
[jc: squashed in git-bisect one noticed by Michael J Gruber]
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-bisect-lk2009.txt

@@ -1321,7 +1321,7 @@ So git bisect is unconditional goodness - and feel free to quote that
 _____________
 
 Acknowledgments
-----------------
+---------------
 
 Many thanks to Junio Hamano for his help in reviewing this paper, for
 reviewing the patches I sent to the Git mailing list, for discussing

Documentation/user-manual.txt

@@ -3424,7 +3424,7 @@ just missing one particular blob version.
 
 [[the-index]]
 The index
------------
+---------
 
 The index is a binary file (generally kept in `.git/index`) containing a
 sorted list of path names, each with permissions and the SHA-1 of a blob