allow disabling .gitignore generation via env

--- cvs-fast-export-1.40/export.c~	2015-11-21 23:16:26.000000000 +0200
+++ cvs-fast-export-1.40/export.c	2016-10-13 21:10:26.557838474 +0300
@@ -491,8 +491,10 @@
 	}
 	if (need_ignores) {
 	    need_ignores = false;
+	    if (!getenv("DISABLE_GITIGNORE")) {
 	    printf("M 100644 inline .gitignore\ndata %zd\n%s\n",
 		   sizeof(CVS_IGNORES)-1, CVS_IGNORES);
+	    }
 	}
 	if (revpairs != NULL && strlen(revpairs) > 0)
 	{