make hashFile support paths with newlines

make hashFile support paths with newlines

git hash-object --stdin-paths is a newline protocol so it cannot
support them. It would help to not use absPath, when the problem
is that the repository itself is in a path with a newline. But,
there's a reason it used absPath, which is that
git hash-object --stdin-paths actually chdirs to the top of the
repository on startup! That is not documented, and I think is a bug
in git.

I considered making the path relative to the top of the repo, but
then what if this is a git bug and gets fixed? git-annex would break
horribly.

So instead, keep the absPath, but when the path contains a newline,
fall back to running git hash-object once per file, which avoids
the problem with newlines and --stdin-paths. It will be slower,
but this is an edge case. (Similar slow code paths are already used
elsewhere when dealing with filenames with newlines and other parts
of git that use line-based protocols.)

Sponsored-by: Dartmouth College's Datalad project