contrib/completion: extract common diff/difftool options

difftool parses its own options and then passes the remaining options
onto diff. As a result, they share common command-line options. Instead
of duplicating the list, use a shared $__git_diff_difftool_options list.

The completion for diff is missing --relative and the completion for
difftool is missing --no-index. Add both of these to the common list.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1691,6 +1691,10 @@ __git_diff_common_options="--stat --numstat --shortstat --summary
 			--textconv --no-textconv
 "
 
+__git_diff_difftool_options="--cached --staged --pickaxe-all --pickaxe-regex
+			--base --ours --theirs --no-index --relative
+			$__git_diff_common_options"
+
 _git_diff ()
 {
 	__git_has_doubledash && return
@@ -1713,10 +1717,7 @@ _git_diff ()
 		return
 		;;
 	--*)
-		__gitcomp "--cached --staged --pickaxe-all --pickaxe-regex
-			--base --ours --theirs --no-index
-			$__git_diff_common_options
-			"
+		__gitcomp "$__git_diff_difftool_options"
 		return
 		;;
 	esac
@@ -1738,11 +1739,7 @@ _git_difftool ()
 		return
 		;;
 	--*)
-		__gitcomp_builtin difftool "$__git_diff_common_options
-					--base --cached --ours --theirs
-					--pickaxe-all --pickaxe-regex
-					--relative --staged
-					"
+		__gitcomp_builtin difftool "$__git_diff_difftool_options"
 		return
 		;;
 	esac