Untangle git_futils_mkdir from git_futils_mkdir_ext - the latter
assumes that we own everything beneath the base, as if it were
being called with a base of the repository or working directory,
and is tailored towards checkout and ensuring that there is no
bogosity beneath the base that must be cleaned up.
This is (at best) slow and (at worst) unsafe in the larger context
of a filesystem where we do not own things and cannot do things like
unlink symlinks that are in our way.
When we don't own a buffer (asize=0) we currently allow the usage of
grow to copy the memory into a buffer we do own. This muddles the
meaning of grow, and lets us be a bit cavalier with ownership semantics.
Don't allow this any more. Usage of grow should be restricted to buffers
which we know own their own memory. If unsure, we must not attempt to
modify it.
When checking out files, we're performing conversion into the user's
native line endings, but we only want to do it for files which have
consistent line endings. Refuse to perform the conversion for mixed-EOL
files.
The CRLF->LF filter is left as-is, as that conversion is considered to be
normalization by git and should force a conversion of the line endings.
There are a few places where we need to join three strings to
assemble a path. This adds a simple join3 function to avoid the
comparatively expensive join_n (which calls strlen on each string
twice).
