Add prefix oriented completions for diff and format-patch commands.

Signed-off-by: NPascal Obry <pascal@obry.net>
Acked-by: NShawn O. Pearce <spearce@spearce.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -639,7 +639,9 @@ _git_diff ()
 			--find-copies-harder --pickaxe-all --pickaxe-regex
 			--text --ignore-space-at-eol --ignore-space-change
 			--ignore-all-space --exit-code --quiet --ext-diff
-			--no-ext-diff"
+			--no-ext-diff
+			--no-prefix --src-prefix= --dst-prefix=
+			"
 		return
 		;;
 	esac
@@ -696,6 +698,7 @@ _git_format_patch ()
 			--full-index --binary
 			--not --all
 			--cover-letter
+			--no-prefix --src-prefix= --dst-prefix=
 			"
 		return
 		;;