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.

Move to the `git_error` name in the internal API for error-related
functions.