pgsql: Teach pgindent to skip files generated by bison or flex automati

Teach pgindent to skip files generated by bison or flex automatically.

If a .c or .h file corresponds to a .y or .l file, skip indenting it.
There's no point in reindenting derived files, and these files tend to
confuse pgindent. (Which probably indicates a bug in BSD indent, but
I can't get excited about trying to fix it.)

For the same reasons, add src/backend/utils/fmgrtab.c to the set of
files excluded by src/tools/pgindent/exclude_file_patterns.

The point of doing this is that it makes it safe to run pgindent over
the tree without doing "make maintainer-clean" first. While these are
not the only derived .c/.h files in the tree, they are the only ones
pgindent fails on. Removing that prerequisite step results in one less
way to mess up a pgindent run, and it's necessary if we ever hope to get
to the ease of running pgindent via "make indent".

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/cea258b63d9c7a6d0a7c5e91e539bb89df4bc078

Modified Files
--------------
src/tools/pgindent/README | 27 +++++++++++++--------------
src/tools/pgindent/exclude_file_patterns | 5 +++--
src/tools/pgindent/pgindent | 9 +++++++++
3 files changed, 25 insertions(+), 16 deletions(-)