Add a simple accessor for workdir paths to get an absolute on-disk path
given a repository and a relative path within it. This is useful to
avoid copy-pasta `git_buf_joinpath` and to ensure that we validate
working directory paths while honoring `core.longpaths` settings.
Our file utils functions all have a "futils" prefix, e.g.
`git_futils_touch`. One would thus naturally guess that their
definitions and implementation would live in files "futils.h" and
"futils.c", respectively, but in fact they live in "fileops.h".
Rename the files to match expectations.
Introduce a repository that contains some paths that were illegal
on PC-DOS circa 1981 (like `aux`, `con`, `com1`) and that in a
bizarre fit of retrocomputing, remain illegal on some "modern"
computers, despite being "new technology".
Introduce some aspirational tests that suggest that we should be
able to cope with trees and indexes that contain paths that
would be illegal on the filesystem, so that we can at least diff
them. Further ensure that checkout will not write a repository
with forbidden paths.
