This commit reworks .gitignore to be more portable depending on the
options used in the PostgreSQL build this module links to, in various
aspects:
- Ignore code generated for LLVM bitcode.
- Dependencies.
- Libraries.
- Coverage files.
- WIN32 and MacOS files.
- Ignore isolation test subdirectory.
- Do *not* ignore regression.diffs and regression.out. This is more
useful to detect if something is broken in the tree, in line with
upstream core practices.
The contents of this file are updated to something closer to what
upstream uses.
The .gitignore file is copied from pg_bigm.
This doesn't contain files generated on Windows development environments for now,
but they will be added when anyone using Windows claim it.
