Linter: Enforce line width

[maxitg]

Changes

• Implements part of #522.
• Adds a script that checks that lines in all files are within the length of 120 chars.
• Calls that script from ./lint.sh so that it is run by CI and git hooks.

Comments

• Markdown files are not enforced because the limit is not respected there at all. A lot of work will need to be done to fix that first.
• CI will fail until existing errors are not fixed. They are fixed in #572 and #569 which will need to be merged first.
• Should be merged after #566 (and reformatting/linting).

Examples

• Here is an example of error messages: example.

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This change is 

[aokellermann]

@maxitg here's some interesting info:

According to sort documentation:

*** WARNING *** The locale specified by the environment affects sort order.  Set LC_ALL=C to get the traditional sort order that uses native byte values.

We can see that git ls-files uses the default locale for sorting (lexicographic order):

diff -q <(git ls-files "${lsfilesOptions[@]}") <(git ls-files "${lsfilesOptions[@]}" | LC_ALL=C sort)

produces no output.

If we specify default locale for comm, there is no need to sort:

LC_ALL=C comm -13 <(git ls-files --deleted) <(git ls-files "${lsfilesOptions[@]}")

works as expected

[maxitg]

The CI was failing because grep with no matches was triggering pipefail. Prevented it now with a || :.