doc: convert \--option to --option

Older versions of AsciiDoc would convert the "--" in
"--option" into an emdash. According to 565e135
(Documentation: quote double-dash for AsciiDoc, 2011-06-29),
this is fixed in AsciiDoc 8.3.0. According to bf17126, we
don't support anything older than 8.4.1 anyway, so we no
longer need to worry about quoting.

Even though this does not change the output at all, there
are a few good reasons to drop the quoting:

  1. It makes the source prettier to read.

  2. We don't quote consistently, which may be confusing when
     reading the source.

  3. Asciidoctor does not like the quoting, and renders a
     literal backslash.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitdiffcore.txt

@@ -28,8 +28,8 @@ The 'git diff-{asterisk}' family works by first comparing two sets of
 files:
 
  - 'git diff-index' compares contents of a "tree" object and the
-   working directory (when '\--cached' flag is not used) or a
-   "tree" object and the index file (when '\--cached' flag is
+   working directory (when '--cached' flag is not used) or a
+   "tree" object and the index file (when '--cached' flag is
    used);
 
  - 'git diff-files' compares contents of the index file and the
@@ -142,7 +142,7 @@ merges these filepairs and creates:
 
 When the "-C" option is used, the original contents of modified files,
 and deleted files (and also unmodified files, if the
-"\--find-copies-harder" option is used) are considered as candidates
+"--find-copies-harder" option is used) are considered as candidates
 of the source files in rename/copy operation.  If the input were like
 these filepairs, that talk about a modified file fileY and a newly
 created file file0: