gnulib bootstrap and git submodules

[Eric Blake]

git 1.6.4 learned 'git submodule add --reference dir' as a means to make
initializing git submodules use (MUCH) less bandwidth and disk space by
borrowing references from an existing on-disk repository, rather than
cloning from scratch.  I would like to modify the bootstrap script to
honor $GNULIB_SRCDIR, if set, as the argument to the --reference
directory, to take advantage of this git feature.  Keep in mind that if
someone sets GNULIB_SRCDIR, it should be to a master repository that does
not get rewound, to avoid issues with the client repositories ever
depending on references that might go stale during garbage collection in
the reference repository (but with gnulib's linear development model, that
is probably not too much of a concern).  Any objections?

Additionally, the m4 bootstrap script (m4 does not use the gnulib
bootstrap script) is able to perform the same operation even with older
git-submodule that does not support the --reference action, by breaking
things down into multiple steps and using git-clone --reference instead.
Is that worth incorporating into my proposed patch for gnulib's bootstrap,
or should we just assume that git 1.6.4 or newer is widespread enough to
not be worth the hassle?

-- 
Don't work too hard, make some time for fun as well!

Eric Blake             address@hidden

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