doc: git-clone fix discrepancy between asciidoc and asciidoctor

Asciidoc.py does not have the concept of generalized roles, whereas
asciidoctor interprets [foo]`blah` as blah with role foo in the
synopsis, making in effect foo disappear in the output. Note that
square brackets not directly followed by an inline markup do not
define a role, which is why we do not have the issue on other parts of
the documentation.

In order to get a consistant result across asciidoctor and
asciidoc.py, the hack is to use the {empty} entity
to split the bracket part from the inline format part.

Signed-off-by: Jean-Noël Avila <[email protected]>

Documentation/git-clone.txt

@@ -13,9 +13,9 @@ SYNOPSIS
 	  [`-l`] [`-s`] [`--no-hardlinks`] [`-q`] [`-n`] [`--bare`] [`--mirror`]
 	  [`-o` _<name>_] [`-b` _<name>_] [`-u` _<upload-pack>_] [`--reference` _<repository>_]
 	  [`--dissociate`] [`--separate-git-dir` _<git-dir>_]
-	  [`--depth` _<depth>_] [`--`[`no-`]`single-branch`] [`--no-tags`]
-	  [++--recurse-submodules++[++=++__<pathspec>__]] [`--`[`no-`]`shallow-submodules`]
-	  [`--`[`no-`]`remote-submodules`] [`--jobs` _<n>_] [`--sparse`] [`--`[`no-`]`reject-shallow`]
+	  [`--depth` _<depth>_] [`--`[`no-`]{empty}`single-branch`] [`--no-tags`]
+	  [++--recurse-submodules++[++=++__<pathspec>__]] [++--++[++no-++]{empty}++shallow-submodules++]
+	  [`--`[`no-`]{empty}`remote-submodules`] [`--jobs` _<n>_] [`--sparse`] [`--`[`no-`]{empty}`reject-shallow`]
 	  [++--filter=++__<filter-spec>__] [`--also-filter-submodules`]] [`--`] _<repository>_
 	  [_<directory>_]