completion: improve bash completion for git-add

Command completion for git-add did not recognize some long-options.
This commits adds completion for all long-options that are mentioned in
the man-page synopsis. In addition, if the user specified `--update` or
`-u`, path completion will only suggest modified tracked files.

Signed-off-by: Cornelius Weig <cornelius.weig@tngtech.com>
Reviewed-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -947,13 +947,17 @@ _git_add ()
 	--*)
 		__gitcomp "
 			--interactive --refresh --patch --update --dry-run
-			--ignore-errors --intent-to-add
+			--ignore-errors --intent-to-add --force --edit --chmod=
 			"
 		return
 	esac
 
-	# XXX should we check for --update and --all options ?
-	__git_complete_index_file "--others --modified --directory --no-empty-directory"
+	local complete_opt="--others --modified --directory --no-empty-directory"
+	if test -n "$(__git_find_on_cmdline "-u --update")"
+	then
+		complete_opt="--modified"
+	fi
+	__git_complete_index_file "$complete_opt"
 }
 
 _git_archive ()