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Fixes #9232

Fixes #8952

Fixes #8637

* use https everywhere

* use https links on the files that are left

Also, removed some broken links and updated redirections.

The avit theme's _git_time_since_commit function was running git twice.
Reduce it with a single call to `git log`, checking the exit code for
success.

$CARETCOLOR was not to actually colour the caret, so change to root user
wouldn't show in the prompt. Now both ▶ and ◀ follow $CARETCOLOR.

Make some colours in the avit theme bold for better visibility in the
default Ubuntu terminal theme.

In commit 729fd53 I incorrectly only changed PROMPT1. Now both ▶ and ◀ follow $CARETCOLOR.

oh-my-zsh avit theme: instead of only showing the last 3 directories in the $PWD shorten the path by removing some middle parts if $PWD becomes too long.

Fix in avit theme: $CARETCOLOR was not to actually colour the caret, so change to root user wouldn't show in the prompt.

Check for at least one commit using a `git log` limit of one.

Improves the performance inside git repos with a large history.