Added sort by commiter date

[agido-malter]

My first idea for #138

feat: add opt-in branch sorting
Add --branch-sort and git-up.branch.sort support using Git for-each-ref sorting semantics.

Preserve the historical alphabetical order by default and deliberately
ignore Git's native branch.sort setting to avoid implicit behavior
changes on upgrade.

[msiemens]

Hey @agido-malter, thanks for the PR, it looks really good! From what I can see, there are only two issues:

1. If a local branch has the same name as a tag (e.g. a branch v1 and a tag v1), %(refname:short) returns heads/v1 instead of v1, so positions[branch.name] crashes with a KeyError. Maybe you could use --format=%(refname:lstrip=2) instead? Could you add a test for that too?
2. An invalid sort key (e.g. --branch-sort=bogus or a typo in git-up.branch.sort) makes git for-each-ref fail inside the constructor. run() then just calls sys.exit(1) without printing anything, so git up silently does nothing. Could we print the error here, like the other constructor errors do?

Also, test_git_up_branch_sort_overrides_native_config sets git-up.branch.sort to refname, which is the same as the default order. So the test would still pass if the config value was ignored entirely, right? Maybe use -committerdate there so we actually test that the config option works?

[agido-malter]

@msiemens thanks for your findings. I fixed them