This change:

* Initializes a few variables that were being read before being
  initialized.
* Includes https://github.com/madler/zlib/pull/393. As such,
  it only works reliably with `-DUSE_BUNDLED_ZLIB=ON`.

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.

We had several occasions where tests for the gitignore had been
added to status::ignore instead of the easier-to-handle
attr::ignore test suite. This most likely resulted from the fact
that the attr::ignore test suite is not easy to discover inside
of the attr folder. Furthermore, ignore being part of the
attributes code is an implementation detail, only, and thus
shouldn't be stressed as much.

Improve this by moving both attr::ignore and status::ignore tests
into a new ignore test suite.

The "ignore.c.bak" file has accidentally been checked in via
commit 191649010 (ignore: test that leading whitespace is
significant, 2019-05-19) and should obviously not be part of our
test suites. Delete it.

Ignore: only treat one leading slash as a root identifier

Comments must have a '#' at the beginning of the line.  For
compatibility with git, '#' after a whitespace is a literal part of the
filename.

Ensure that leading whitespace is treated as being part of the filename,
eg ` foo` in an ignore file indicates that a file literally named ` foo`
is ignored.

Ensure that we can read and parse an ignore file with a UTF8 BOM.