add -p: fix 2.17.0-rc* regression due to moved code

Fix a regression in 88f6ffc1 ("add -p: only bind search key if
there's more than one hunk", 2018-02-13) which is present in
2.17.0-rc*, but not 2.16.0.

In Perl, regex variables like $1 always refer to the last regex
match. When the aforementioned change added a new regex match between
the old match and the corresponding code that was expecting $1, the $1
variable would always be undef, since the newly inserted regex match
doesn't have any captures.

As a result the "/" feature to search for a string in a hunk by regex
completely broke, on git.git:

    $ perl -pi -e 's/Git/Tig/g' README.md
    $ ./git --exec-path=$PWD add -p
    [..]
    Stage this hunk [y,n,q,a,d,j,J,g,/,s,e,?]? s
    Split into 4 hunks.
    [...]
    Stage this hunk [y,n,q,a,d,j,J,g,/,s,e,?]? /Many
    Use of uninitialized value $1 in string eq at /home/avar/g/git/git-add--interactive line 1568, <STDIN> line 1.
    search for regex? Many

I.e. the initial "/regex" command wouldn't work, and would always emit
a warning and ask again for a regex, now it works as intended again.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-add--interactive.perl

@@ -1492,7 +1492,7 @@ sub patch_update_file {
 					error_msg __("No other hunks to search\n");
 					next;
 				}
-				if ($1 eq "") {
+				if ($regex eq "") {
 					print colored $prompt_color, __("search for regex? ");
 					$regex = <STDIN>;
 					if (defined $regex) {