matcher: fix issues regex flag contained in pattern (issue6759) Python 3.11 is now enforcing that flag must be at the beginning of the regex This creates a serious regression for people using Python 3.11 with an hgignore using flag in a "relre" pattern. We now detect any flags in such pattern and "prepend" our ".*" pattern after them. In addition, we now insert the flag in the regexp to only affect the pattern we are rewriting. Otherwise, the regex built from the combined pattern would these flags in the middle of it anyway. As a side effect of this last change, we fix a bug… before this change regex flag in a pattern would affect all combined patterns. That was bad and is not longer the case. The Rust code needs to be updated to fix that very bug, but we will do it in another changeset.