pull: do nothing on --dry-run

Pull was never meant to take --dry-run at all. However, it
passes unknown arguments to git-fetch, which does do a
dry-run. Unfortunately, pull then attempts to merge whatever
cruft was in FETCH_HEAD (which the dry-run fetch will not
have written to).

Even though we never advertise --dry-run as something that
should work, it is still worth being defensive because:

  1. Other commands (including fetch) take --dry-run, so a
     user might try it.

  2. Rather than simply producing an error, it actually
     changes the repository in totally unexpected ways.

This patch makes "pull --dry-run" equivalent to "fetch
--dry-run".

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-pull.sh

@@ -43,6 +43,7 @@ merge_args=
 curr_branch=$(git symbolic-ref -q HEAD)
 curr_branch_short="${curr_branch#refs/heads/}"
 rebase=$(git config --bool branch.$curr_branch_short.rebase)
+dry_run=
 while :
 do
 	case "$1" in
@@ -104,6 +105,9 @@ do
 	--no-r|--no-re|--no-reb|--no-reba|--no-rebas|--no-rebase)
 		rebase=false
 		;;
+	--d|--dr|--dry|--dry-|--dry-r|--dry-ru|--dry-run)
+		dry_run=--dry-run
+		;;
 	-h|--h|--he|--hel|--help)
 		usage
 		;;
@@ -216,7 +220,8 @@ test true = "$rebase" && {
 	done
 }
 orig_head=$(git rev-parse -q --verify HEAD)
-git fetch $verbosity $progress --update-head-ok "$@" || exit 1
+git fetch $verbosity $progress $dry_run --update-head-ok "$@" || exit 1
+test -z "$dry_run" || exit 0
 
 curr_head=$(git rev-parse -q --verify HEAD)
 if test -n "$orig_head" && test "$curr_head" != "$orig_head"