Remove dependencies on Hash::Merge and Clone::Choose

stow 2.3.0 added external runtime dependencies on Hash::Merge and
Clone::Choose. Historically stow hasn't had runtime dependencies other
than Perl itself, which is a useful property if you're managing the
installation of Perl using stow; the bootstrapping instructions in
stow's manual would need updating to describe how to install these two
modules (and any dependencies they have now or in the future) as well.

However, Hash::Merge is much more general than stow actually needs, so
replace the merge() call with a few lines of equivalent code -- this
avoids the external dependencies, and is clearer than the merge()
call.

Many thanks to Adam Sampson for this patch:

https://lists.gnu.org/archive/html/bug-stow/2019-06/msg00001.html

t/rc_options.t

@@ -22,7 +22,7 @@
 use strict;
 use warnings;
 
-use Test::More tests => 33;
+use Test::More tests => 34;
 
 use testutil;
 
@@ -120,16 +120,19 @@ is($options->{dir}, "$ABS_TEST_DIR/stow"
    => "-d from \$HOME/.stowrc");
 
 #
-# Test ~/.stowrc file is overridden by .stowrc in cwd.
+# Test that some but not all options ~/.stowrc file are overridden by
+# .stowrc in cwd.
 #
 local @ARGV = ('dummy');
 make_file($HOME_RC_FILE, <<HERE);
     -d $ABS_TEST_DIR/stow-will-be-overridden
     --target $ABS_TEST_DIR/target-will-be-overridden
+    --defer=info
 HERE
 make_file($CWD_RC_FILE, <<HERE);
     -d $ABS_TEST_DIR/stow
     --target $ABS_TEST_DIR/target
+    --defer=man
 HERE
 
 ($options, $pkgs_to_delete, $pkgs_to_stow) = process_options();
@@ -137,6 +140,8 @@ is($options->{target},  "$ABS_TEST_DIR/target"
    => "--target overridden by \$PWD/.stowrc");
 is($options->{dir}, "$ABS_TEST_DIR/stow"
    => "-d overridden \$PWD/.stowrc");
+is_deeply($options->{defer}, [qr(\Ainfo), qr(\Aman)],
+          'defer man and info');
 unlink($CWD_RC_FILE) or die "Failed to unlink $CWD_RC_FILE";
 
 #