coverity: model functions printing into git_buf

The `git_buf` structure seems to be too complicated to correctly
grasp for Coverity. As such, add simpler models trying to guide
Coverity and remove false positives related to these functions.

script/user_model.c

@@ -6,8 +6,11 @@
  */
 
 void *realloc(void *ptr, size_t size);
+void *memmove(void *dest, const void *src, size_t n);
 size_t strlen(const char *s);
 
+typedef struct va_list_str *va_list;
+
 typedef struct git_vector {
 	void **contents;
 	size_t length;
@@ -35,3 +38,38 @@ int git_buf_len(const struct git_buf *buf)
 {
 	return strlen(buf->ptr);
 }
+
+int git_buf_vprintf(git_buf *buf, const char *format, va_list ap)
+{
+    char ch, *s;
+    size_t len;
+
+    __coverity_string_null_sink__(format);
+    __coverity_string_size_sink__(format);
+
+    ch = *format;
+    ch = *(char *)ap;
+
+    buf->ptr = __coverity_alloc__(len);
+    __coverity_writeall__(buf->ptr);
+    buf->size = len;
+
+    return 0;
+}
+
+int git_buf_put(git_buf *buf, const char *data, size_t len)
+{
+    buf->ptr = __coverity_alloc__(buf->size + len + 1);
+    memmove(buf->ptr + buf->size, data, len);
+    buf->size += len;
+    buf->ptr[buf->size + len] = 0;
+    return 0;
+}
+
+int git_buf_set(git_buf *buf, const void *data, size_t len)
+{
+    buf->ptr = __coverity_alloc__(len + 1);
+    memmove(buf->ptr, data, len);
+    buf->size = len + 1;
+    return 0;
+}