log: add log.excludeDecoration config option

log: add log.excludeDecoration config option

In 'git log', the --decorate-refs-exclude option appends a pattern
to a string_list. This list is used to prevent showing some refs
in the decoration output, or even by --simplify-by-decoration.

Users may want to use their refs space to store utility refs that
should not appear in the decoration output. For example, Scalar [1]
runs a background fetch but places the "new" refs inside the
refs/scalar/hidden/<remote>/* refspace instead of refs/<remote>/*
to avoid updating remote refs when the user is not looking. However,
these "hidden" refs appear during regular 'git log' queries.

A similar idea to use "hidden" refs is under consideration for core
Git [2].

Add the 'log.excludeDecoration' config option so users can exclude
some refs from decorations by default instead of needing to use
--decorate-refs-exclude manually. The config value is multi-valued
much like the command-line option. The documentation is careful to
point out that the config value can be overridden by the
--decorate-refs option, even though --decorate-refs-exclude would
always "win" over --decorate-refs.

Since the 'log.excludeDecoration' takes lower precedence to
--decorate-refs, and --decorate-refs-exclude takes higher
precedence, the struct decoration_filter needed another field.
This led also to new logic in load_ref_decorations() and
ref_filter_match().

There are several tests in t4202-log.sh that test the
--decorate-refs-(include|exclude) options, so these are extended.
Since the expected output is already stored as a file, most tests
could simply replace a "--decorate-refs-exclude" option with an
in-line config setting. Other tests involve the precedence of
the config option compared to command-line options and needed more
modification.

[1] https://github.com/microsoft/scalar
[2] https://lore.kernel.org/git/77b1da5d3063a2404cd750adfe3bb8be9b6c497d.1585946894.git.gitgitgadget@gmail.com/

Helped-by: Junio C Hamano <gister@pobox.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>