The $Id$ keywords were used in Subversion where they can be substituted
with filename, last revision number change, last changed date, and last
user who changed it.
In Git this functionality is different and can be done with Git attribute
ident. These need to be defined manually for each file in the
.gitattributes file and are afterwards replaced with 40-character
hexadecimal blob object name which is based only on the particular file
contents.
This patch simplifies handling of $Id$ keywords by removing them since
they are not used anymore.
This breaks user-space filters (for the time being), and those
weird convert.* filters in ext/standard/filters.c
The filters stack has been separated into one chain for read and one chain
for write.
The user-space stream_filter_append() type functions currently only operate
on the read chain. They need extending to work with the write chain too.