diff,difftool: Don't use the {0,2} notation in usage strings

This was the only occurence of that usage, and square brackets are
sufficient and already well-established for that purpose.

Signed-off-by: Štěpán Němec <stepnem@gmail.com>
Acked-by: Sverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-diff.txt

@@ -8,7 +8,7 @@ git-diff - Show changes between commits, commit and working tree, etc
 
 SYNOPSIS
 --------
-'git diff' [<common diff options>] <commit>{0,2} [--] [<path>...]
+'git diff' [<common diff options>] [<commit> [<commit>]] [--] [<path>...]
 
 DESCRIPTION
 -----------

Documentation/git-difftool.txt

@@ -7,7 +7,7 @@ git-difftool - Show changes using common diff tools
 
 SYNOPSIS
 --------
-'git difftool' [<options>] <commit>{0,2} [--] [<path>...]
+'git difftool' [<options>] [<commit> [<commit>]] [--] [<path>...]
 
 DESCRIPTION
 -----------

builtin/diff.c

@@ -22,7 +22,7 @@ struct blobinfo {
 };
 
 static const char builtin_diff_usage[] =
-"git diff <options> <rev>{0,2} -- <path>*";
+"git diff [<options>] [<commit> [<commit>]] [--] [<path>...]";
 
 static void stuff_change(struct diff_options *opt,
 			 unsigned old_mode, unsigned new_mode,