allow disabling .gitignore generation via env

Upstream: https://gitlab.com/esr/cvs-fast-export/issues/13

--- cvs-fast-export-1.66/export.c.orig	2024-04-07 15:42:44.128586210 +0200
+++ cvs-fast-export-1.66/export.c	2024-04-07 15:43:38.798290039 +0200
@@ -581,8 +581,10 @@ static void export_commit(git_commit *co
 		}
 		if (need_ignores) {
 			need_ignores = false;
+			if (!getenv("DISABLE_GITIGNORE")) {
 			printf("M 100644 inline .gitignore\ndata %d\n%s\n",
 			       (int)sizeof(CVS_IGNORES) - 1, CVS_IGNORES);
+			}
 		}
 		if (revpairs != NULL && strlen(revpairs) > 0) {
 			if (opts->revision_map) {