This patch forces write_file_if_contents_differ/2 to use raw bytes as
when writing files. This fixes an issue where UTF-8 characters are
processed and written back as ISO-8859-1 into the file, which makes it
impossible to use UTF-8 characters in .app.src files.

Linking message will only be printed the first time the link or copy
actually happens.

Note:

rebar_file_utils:symlink_or_copy will now return 'exists'
instead of ok, if it did nothing because the link exists.

Nothing was checking the return value yet, so seemed reasonable.

Basic escaping is done only. Fancy hex sequences are not covered, but
this should otherwise take care of the most common issues.

Fixes #497

(in `rebar_file_utils`)

While at it, improve the error message printed by rebar_utils:sh/2.

'Erlang' projects that do not contain any erlang files (Joxa, LFE,
Elixir, etc) break the eunit task. It attempts to copy an empty list
of source files to the .eunit directory. This change makes copying an
empty list a simple no-op.

Add flags to rebar_utils:sh to control output and error handling.
Replace calls to os:cmd with calls to rebar_utils:sh.

Modify rm_rf and cp_r to work when {win32,_} = os:type().
Simplify rm_rf to only accept one filename, directoryname or wildcard.
Add unit tests to ensure a similar behaviour on windows and unix.

Thanks to tuncer for guidance and feedback.

mkdir_p/1 and ln_sf/2.

to the OTP bin dir.