Git.pm: Make File::Spec and File::Temp requirement lazy

This will ensure that the API at large is accessible to nearly
all Perl versions, while only the temp file caching API is tied to
the File::Temp and File::Spec modules being available.

Signed-off-by: Marcus Griep <marcus@griep.us>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

perl/Git.pm

@@ -100,8 +100,6 @@ use Carp qw(carp croak); # but croak is bad - throw instead
 use Error qw(:try);
 use Cwd qw(abs_path);
 use IPC::Open2 qw(open2);
-use File::Temp ();
-require File::Spec;
 use Fcntl qw(SEEK_SET SEEK_CUR);
 }
 
@@ -1009,6 +1007,8 @@ sub temp_release {
 sub _temp_cache {
 	my ($name) = @_;
 
+	_verify_require();
+
 	my $temp_fd = \$TEMP_FILES{$name};
 	if (defined $$temp_fd and $$temp_fd->opened) {
 		if ($TEMP_LOCKS{$$temp_fd}) {
@@ -1031,6 +1031,11 @@ sub _temp_cache {
 	$$temp_fd;
 }
 
+sub _verify_require {
+	eval { require File::Temp; require File::Spec; };
+	$@ and throw Error::Simple($@);
+}
+
 =item temp_reset ( FILEHANDLE )
 
 Truncates and resets the position of the C<FILEHANDLE>.