devel/qgit: update to 2.13 (latest upstream) and Qt6

Release notes are at
https://github.com/tibirna/qgit/releases/tag/qgit-2.13
but basically it's a bugfix release for Qt6-porting-issues.
We skipped .10, .11, and .12 which had some new features,
small UI / user-experience improvements.

PR: 289588
Reported-by: diizzy@