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After addition of "set -u" in the last commit, we should initialize
all used variables.

Without this change, we se the "No AE updates detected" message even
when there were updates.

Pushing sequentially is unnecessarily slow.

See the comments in the script for more details.

This commit changes how is the hook configured. Instead of having
brutegit remote and brutegit.glob values in git configuration, now
everything is configured in .brutegit-ae-sync file in the root
directory of the repo. See this repo for an example.

The reason for this breaking change is that we have to support
multiple remotes and that we want the content of the AE repos
configurable via the course repo itself (i.e. via .brutegit-ae-sync
file in the repo).

Store the whole match pattern in a variable so that Emacs indenting
code is not confused.

- Use local variables instead of global ones
- Make branch a variable

Also add systemd .service file to start the webhook script.

See the comment in the code.