pretty: add %(decorate[:<options>]) format

Add %(decorate[:<options>]) format that lists ref names similarly to the
%d format, but which allows the otherwise fixed prefix, suffix and
separator strings to be customized. Omitted options default to the
strings used in %d.

Rename expand_separator() function used to expand %x literal formatting
codes to expand_string_arg(), as it is now used on strings other than
separators.

Examples:
- %(decorate) is equivalent to %d.
- %(decorate:prefix=,suffix=) is equivalent to %D.
- %(decorate:prefix=[,suffix=],separator=%x3B) produces a list enclosed
in square brackets and separated by semicolons.

Test the format in t4205-log-pretty-formats.sh and document it in
pretty-formats.txt.

Signed-off-by: Andy Koppe <andy.koppe@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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