Fix regression in rubygem unpacking (#2040)

Externalizing the source unpack in commit
cd5d667 required changing the way
rubygems are unpacked, and now the .gemspec file gets unpacked into
different place. Oops. Reported and initial patch by Vit Ondruch.

Fixes: #2040

tools/rpmuncompress.c

@@ -97,12 +97,13 @@ static char *doUntar(const char *fn)
 	if (needtar) {
 	    rasprintf(&buf, "%s '%s' | %s %s -", zipper, fn, tar, taropts);
 	} else if (at->compressed == COMPRESSED_GEM) {
-	    size_t nvlen = strlen(fn) - 3;
+	    const char *bn = basename(fn);
+	    size_t nvlen = strlen(bn) - 3;
 	    char *gem = rpmGetPath("%{__gem}", NULL);
 	    char *gemspec = NULL;
 	    char gemnameversion[nvlen];
 
-	    rstrlcpy(gemnameversion, fn, nvlen);
+	    rstrlcpy(gemnameversion, bn, nvlen);
 	    gemspec = rpmGetPath("", gemnameversion, ".gemspec", NULL);
 
 	    rasprintf(&buf, "%s '%s' && %s spec '%s' --ruby > '%s'",