git-send-email: Respect core.hooksPath setting

get-send-email currently makes the assumption that the
'sendemail-validate' hook exists inside of the repository.

Since the introduction of 'core.hooksPath' configuration option in
867ad08a26 (hooks: allow customizing where the hook directory is,
2016-05-04), this is no longer true.

Instead of assuming a hardcoded repo relative path, query
git for the actual path of the hooks directory.

Signed-off-by: Robert Foss <robert.foss@linaro.org>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

perl/Git.pm

@@ -619,6 +619,19 @@ Return path to the git repository. Must be called on a repository instance.
 
 sub repo_path { $_[0]->{opts}->{Repository} }
 
+=item hooks_path ()
+
+Return path to the hooks directory. Must be called on a repository instance.
+
+=cut
+
+sub hooks_path {
+	my ($self) = @_;
+
+	my $dir = $self->command_oneline('rev-parse', '--git-path', 'hooks');
+	my $abs = abs_path($dir);
+	return $abs;
+}
 
 =item wc_path ()