completion: no-op refactoring of diff completion

The "git diff" completion function punts very early when it sees
"--" on the command line, since it is a sign that options or
revisions can appear and the current completion does not need to do
anything "git diff" specific. By returning, it lets Bash default
action that completes the names of the files in $PWD to kick in.

In preparation for the next step to change what happens when we
"punt", arrange the code flow to avoid this early return.  The
behaviour at this step is unchanged, but the control flow just
falls straight to the end.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

diff --git a/contrib/completion/git-completion.bash b/contrib/completion/git-completion.bash
index e875787..a61b6ed 100644
--- a/contrib/completion/git-completion.bash
+++ b/contrib/completion/git-completion.bash
@@ -1947,35 +1947,40 @@
 
 _git_diff ()
 {
-	__git_has_doubledash && return
-
-	case "$cur" in
-	--diff-algorithm=*)
-		__gitcomp "$__git_diff_algorithms" "" "${cur##--diff-algorithm=}"
-		return
-		;;
-	--submodule=*)
-		__gitcomp "$__git_diff_submodule_formats" "" "${cur##--submodule=}"
-		return
-		;;
-	--color-moved=*)
-		__gitcomp "$__git_color_moved_opts" "" "${cur##--color-moved=}"
-		return
-		;;
-	--color-moved-ws=*)
-		__gitcomp "$__git_color_moved_ws_opts" "" "${cur##--color-moved-ws=}"
-		return
-		;;
-	--ws-error-highlight=*)
-		__gitcomp "$__git_ws_error_highlight_opts" "" "${cur##--ws-error-highlight=}"
-		return
-		;;
-	--*)
-		__gitcomp "$__git_diff_difftool_options"
-		return
-		;;
-	esac
-	__git_complete_revlist_file
+	if ! __git_has_doubledash; then
+		case "$cur" in
+		--diff-algorithm=*)
+			__gitcomp "$__git_diff_algorithms" \
+				"" "${cur##--diff-algorithm=}"
+			return
+			;;
+		--submodule=*)
+			__gitcomp "$__git_diff_submodule_formats" \
+				"" "${cur##--submodule=}"
+			return
+			;;
+		--color-moved=*)
+			__gitcomp "$__git_color_moved_opts" \
+				"" "${cur##--color-moved=}"
+			return
+			;;
+		--color-moved-ws=*)
+			__gitcomp "$__git_color_moved_ws_opts" \
+				"" "${cur##--color-moved-ws=}"
+			return
+			;;
+		--ws-error-highlight=*)
+			__gitcomp "$__git_ws_error_highlight_opts" \
+				"" "${cur##--ws-error-highlight=}"
+			return
+			;;
+		--*)
+			__gitcomp "$__git_diff_difftool_options"
+			return
+			;;
+		esac
+		__git_complete_revlist_file
+	fi
 }
 
 __git_mergetools_common="diffuse diffmerge ecmerge emerge kdiff3 meld opendiff