tests: avoid a race in tail --retry testing

Prompted by the continuous integration build failure at:
http://hydra.nixos.org/build/5221053

* tests/tail-2/retry.sh: Ensure the 'out' file is truncated,
as it's used to arbitrate the run order of commands.
Relying on the truncation in the background tail command
is racy because the truncation can occur after the fork
of the background shell and thus wait4lines would not wait
for output to occur in 'out', which would mean that the
'missing' file could be populated by the time tail(1)
gets to process it.
build: update gnulib submodule; also bootstrap to latest

Notes tests/init.sh is still in sync with gnulib.

* bootstrap: Update to latest.
* gnulib: Update avoiding gnulib-test failures.
doc: mention prior stat change

For a file of size 1234 bytes, commit ca9aa759 had the side effect
of changing 'stat -c "%'s" file' from outputting "?s" to the nicer
"1,234", depending on locale.  This is worth mentioning in the NEWS.

Resolves part of http://bugs.gnu.org/14556.

* NEWS: Mention 8.7 improvement in stat.
* cfg.mk (old_NEWS_hash): Adjust accordingly.
doc: clarify the description of du --separate-dirs

* src/du.c (usage): Clarify that --separate-dirs doesn't exclude
all directories.
* doc/coreutils.texi (du invocation): Avoid implying that -S
excludes the size of any non directory entries for a directory.
Also don't mention st_size as it's dependent on --apparent-size.
Reported by C de-Avillez in <https://launchpad.net/bugs/1187044>
maint: update gitignore entries

* .gitignore: Exclude the generated doc/manual directory,
and also the src/fs-latest-magic.h file referenced in README-release.