[postgis-devel] make_dist.sh improvements

[Sandro Santilli]

I've added a couple of changes to the make_dist.sh script:

 1) Added -b switch to specify a branch name

 2) Automatic use of GIT when called from a git repository

Existing release workflow should still work fine, but as an added
bonus we should now be able to avoid hitting the network when having
already a copy of the repository (common with git-svn or clonese
of the official mirrors), and we now have a way to create tarballs
from a branch other than "trunk" (for 2.2.x release candidates,
for example).

Let me know if you have any problem, thanks.

--strk; 

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