[PATCH 0/8] Rerolling sb/submodule-parallel-fetch for the time after 2.7

I am sending out a new version for replacing sb/submodule-parallel-fetch for the time after the 2.7 release.

The content are 
 * all patches as in the branch sb/submodule-parallel-fetch
 * inlcuding the fixups as suggested by Hannes, 
 * write a message to the debug log for better testing and debugging purposes
  (a patch cherry picked from the series which is supposed to build on top of this)

The patches themselves were rebased such that there are no fixup commits any more, but we get things right the first time.

The commit message of "run-command: add an asynchronous parallel child processor" has slightly been updated to mention the fact that we don't want to use waitpid(-1) but rather use the assumption of child's stderr living as long as the child itself.

Thanks, Stefan

Jonathan Nieder (1):
  submodule.c: write "Fetching submodule <foo>" to stderr

Stefan Beller (7):
  xread: poll on non blocking fds
  xread_nonblock: add functionality to read from fds without blocking
  strbuf: add strbuf_read_once to read without blocking
  sigchain: add command to pop all common signals
  run-command: add an asynchronous parallel child processor
  fetch_populated_submodules: use new parallel job processing
  submodules: allow parallel fetching, add tests and documentation

 Documentation/fetch-options.txt |   7 +
 builtin/fetch.c                 |   6 +-
 builtin/pull.c                  |   6 +
 git-compat-util.h               |   1 +
 run-command.c                   | 335 ++++++++++++++++++++++++++++++++++++++++
 run-command.h                   |  80 ++++++++++
 sigchain.c                      |   9 ++
 sigchain.h                      |   1 +
 strbuf.c                        |  11 ++
 strbuf.h                        |   8 +
 submodule.c                     | 141 +++++++++++------
 submodule.h                     |   2 +-
 t/t0061-run-command.sh          |  53 +++++++
 t/t5526-fetch-submodules.sh     |  71 ++++++---
 test-run-command.c              |  55 ++++++-
 wrapper.c                       |  35 ++++-
 16 files changed, 747 insertions(+), 74 deletions(-)