doc: typeset '--' as literal

This was obtained with:

  perl -pi -e "s/'--'/\`--\`/g" *.txt

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-fast-import.txt

@@ -1054,7 +1054,7 @@ relative-marks::
 no-relative-marks::
 force::
 	Act as though the corresponding command-line option with
-	a leading '--' was passed on the command line
+	a leading `--` was passed on the command line
 	(see OPTIONS, above).
 
 import-marks::
@@ -1105,7 +1105,7 @@ options the user may specify to git fast-import itself.
 
 The `<option>` part of the command may contain any of the options
 listed in the OPTIONS section that do not change import semantics,
-without the leading '--' and is treated in the same way.
+without the leading `--` and is treated in the same way.
 
 Option commands must be the first commands on the input (not counting
 feature commands), to give an option command after any non-option

Documentation/git-filter-branch.txt

@@ -197,7 +197,7 @@ to other tags will be rewritten to point to the underlying commit.
 <rev-list options>...::
 	Arguments for 'git rev-list'.  All positive refs included by
 	these options are rewritten.  You may also specify options
-	such as `--all`, but you must use '--' to separate them from
+	such as `--all`, but you must use `--` to separate them from
 	the 'git filter-branch' options. Implies <<Remap_to_ancestor>>.
 
 

Documentation/rev-list-options.txt

@@ -193,7 +193,7 @@ endif::git-rev-list[]
 
 --stdin::
 	In addition to the '<commit>' listed on the command
-	line, read them from the standard input. If a '--' separator is
+	line, read them from the standard input. If a `--` separator is
 	seen, stop reading commits and start reading paths to limit the
 	result.