Write the system ARG_MAX to an autogenerated config.py at configure time.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Add a batchpipe() command to helpers that behaves somewhat like xargs(1).

Add batchpipe(), which will yield the output produced by calling a
given external command with a given list of arguments.

The resulting output may be provided in chunks, from multiple
invocations of the command, if the limits imposed by ARG_MAX make that
necessary.

See http://www.in-ulm.de/~mascheck/various/argmax/ for details, but
note that batchpipe() takes the additional precaution of adding room
for the argv pointers in addition to the envp pointers.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Retrieve the dates for all branches with one bulk git call in the VFS.

Instead of calling "git rev-list" once per branch to get the branch
tip dates for the VFS branch list, make a single call to "git show"
using a suitable pretty format, and pass all the branch refs as
arguments.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>