chore: update gix to 0.73

[blinxen]

This PR tries to update gix to version 0.73.

The changes were needed because of:

• SignatureRef.time method now returns a &str. There is a SignatureRef.time() to get Time back. This can however fail and return an Error. I replaced all instances where time was accessed directly with time()?. In one case, I couldn't use the ? operator because the method did not return a Result. How do you want this to be handled? Is there some default value that can be used here? The compile error looks like this:

> cargo b
   Compiling stgit v2.5.3 (/home/hka/dev/opensource/stgit)
error[E0308]: mismatched types
  --> src/ext/signature.rs:49:24
   |
49 |             .unwrap_or(self.time);
   |              --------- ^^^^^^^^^ expected `Time`, found `&str`
   |              |
   |              arguments to this method are incorrect
   |
help: the return type of this call is `&str` due to the type of the argument passed
  --> src/ext/signature.rs:46:20
   |
46 |           let time = matches
   |  ____________________^
47 | |             .get_one::<gix::date::Time>("authdate")
48 | |             .copied()
49 | |             .unwrap_or(self.time);
   | |________________________---------^
   |                          |
   |                          this argument influences the return type of `unwrap_or`
note: method defined here
  --> /builddir/build/BUILD/rust-1.88.0-build/rustc-1.88.0-src/library/core/src/option.rs:1023:12

• SignatureRef.to_owned() can now fail and needs to handled accordingly (by using the ? operator).
• RepositoryExtended.commit_ex() wants a SignatureRef but Signature was passed. I assume that because of the updated somewhere Signature is returned instead of SignatureRef. I added a couple of Signature.to_ref calls where needed.