Fix multi-engine template resolution

When adding a change to multiple engines, Sqitch would randomly pick one
engine's templates or the others and create them all for the same
engine. In other words, adding a change to both "sqlite" and "pg"
targets would result in all change files generated from templates from
only one of those engines -- whichever it happened to find first.

Due to the use of a hash reference for templates in add.pm. Fix it by
making a copy of the hash instead. This leads to duplication of effort
when multiple targets use the same engine, but the cost is generally low
relative to how often `add` is called -- and likely is measured in
milliseconds.

Resolves #795.

Changes

@@ -17,6 +17,9 @@ Revision history for Perl extension App::Sqitch
      - Removed the duplicate change name from the output of the list of
        changes to be deployed or reverted with `-VV`. Thanks to Erik Wienhold
        for the PR (#787)!
+     - Fixed invalid template resolution when adding a singe to multiple
+       engines at once. Thanks to Christian Riedel for the detailed bug report
+       (#795)!
 
 1.4.0 2023-08-01T23:37:30Z
      - Fixed Snowflake warehouse and role setup to properly quote identifiers

lib/App/Sqitch/Command/add.pm

@@ -121,7 +121,7 @@ sub _config_templates {
 sub all_templates {
     my ($self, $name) = @_;
     my $config = $self->sqitch->config;
-    my $tmpl   = $self->templates;
+    my $tmpl   = { %{ $self->templates } };
 
     # Read all the template directories.
     for my $dir (

t/add.t

@@ -3,7 +3,7 @@
 use strict;
 use warnings;
 use utf8;
-use Test::More tests => 242;
+use Test::More tests => 243;
 #use Test::More 'no_plan';
 use App::Sqitch;
 use App::Sqitch::Target;
@@ -301,7 +301,14 @@ is_deeply $add->all_templates($tname), {
     deploy => file('etc/templates/deploy/pg.tmpl'),
     revert => file('etc/templates/revert/pg.tmpl'),
     verify => file('etc/templates/verify/pg.tmpl'),
-}, 'Should find all templates in directory';
+}, 'Should find all pg templates in directory';
+
+# Make sure it works for a second name.
+is_deeply $add->all_templates('sqlite'), {
+    deploy => file('etc/templates/deploy/sqlite.tmpl'),
+    revert => file('etc/templates/revert/sqlite.tmpl'),
+    verify => file('etc/templates/verify/sqlite.tmpl'),
+}, 'Should find all sqlite templates in directory';
 
 # Now let it find the templates in the user dir.
 $usrdir = dir 'etc';