userdiff: support Bash

userdiff: support Bash

Support POSIX, bashism and mixed function declarations, all four
compound command types, trailing comments and mixed whitespace.

Even though Bash allows locale-dependent characters in function names
<https://unix.stackexchange.com/a/245336/3645>, only detect function
names with characters allowed by POSIX.1-2017
<https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap03.html#tag_03_235>
for simplicity. This should cover the vast majority of use cases, and
produces system-agnostic results.

Since a word pattern has to be specified, but there is no easy way to
know the default word pattern, use the default `IFS` characters for a
starter. A later patch can improve this.

Signed-off-by: Victor Engmark <victor@engmark.name>
Acked-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>