coverity: add user model

The static analysis engine coverity allows for user models
overriding how it treats functions when analyzing code. Like
this, one can greatly reduce the rate of false positives and thus
make it easier to spot actual errors.

Add a user model that overrides function models for `git_buf_len`
and `git_vector_insert`, which together amount for a majority of
false positives.

script/user_model.c

+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+
+void *realloc(void *ptr, size_t size);
+size_t strlen(const char *s);
+
+typedef struct git_vector {
+	void **contents;
+	size_t length;
+} git_vector;
+
+typedef struct git_buf {
+	char *ptr;
+	size_t asize, size;
+} git_buf;
+
+int git_vector_insert(git_vector *v, void *element)
+{
+	if (!v)
+		__coverity_panic__();
+
+	v->contents = realloc(v->contents, ++v->length);
+	if (!v->contents)
+		__coverity_panic__();
+	v->contents[v->length] = element;
+
+	return 0;
+}
+
+int git_buf_len(const struct git_buf *buf)
+{
+	return strlen(buf->ptr);
+}