git-svn: lazy load some modules

We can delay loading some modules until we need them for uncommon
code paths.  For example, persistent memoization is not often
needed, so we can avoid loading the modules for it until we
encounter svn::mergeinfo during fetch.

This gives a tiny reduction in syscalls (from 15641 to 15305) when
running "git svn info" and counting via "strace -fc".  Further,
more invasive work will be needed to noticeably improve performance.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
diff --git a/perl/Git/SVN.pm b/perl/Git/SVN.pm
index 8e4af71..afa562c 100644
--- a/perl/Git/SVN.pm
+++ b/perl/Git/SVN.pm
@@ -9,10 +9,8 @@
 	    $_use_log_author $_add_author_from $_localtime/;
 use Carp qw/croak/;
 use File::Path qw/mkpath/;
-use File::Copy qw/copy/;
 use IPC::Open3;
 use Memoize;  # core since 5.8.0, Jul 2002
-use Memoize::Storable;
 use POSIX qw(:signal_h);
 
 use Git qw(
@@ -32,11 +30,7 @@
 	add_path_to_url
 );
 
-my $can_use_yaml;
-BEGIN {
-	$can_use_yaml = eval { require Git::SVN::Memoize::YAML; 1};
-}
-
+my $memo_backend;
 our $_follow_parent  = 1;
 our $_minimize_url   = 'unset';
 our $default_repo_id = 'svn';
@@ -1578,7 +1572,16 @@
 	my $hash = shift;
 	my $path = shift;
 
-	if ($can_use_yaml) {
+	unless ($memo_backend) {
+		if (eval { require Git::SVN::Memoize::YAML; 1}) {
+			$memo_backend = 1;
+		} else {
+			require Memoize::Storable;
+			$memo_backend = -1;
+		}
+	}
+
+	if ($memo_backend > 0) {
 		tie %$hash => 'Git::SVN::Memoize::YAML', "$path.yaml";
 	} else {
 		tie %$hash => 'Memoize::Storable', "$path.db", 'nstore';
@@ -2188,8 +2191,9 @@
 	# both of these options make our .rev_db file very, very important
 	# and we can't afford to lose it because rebuild() won't work
 	if ($self->use_svm_props || $self->no_metadata) {
+		require File::Copy;
 		$sync = 1;
-		copy($db, $db_lock) or die "rev_map_set(@_): ",
+		File::Copy::copy($db, $db_lock) or die "rev_map_set(@_): ",
 					   "Failed to copy: ",
 					   "$db => $db_lock ($!)\n";
 	} else {