[1.8.0] reorganize the mess that the source tree has become

To me the source tree looks highly untidy to me.

We do have subdirectories for documentation, tests, contributions, etc. But a sizeable part of the tree is just a big splat of source files dumped right in the root of the tree.

So I'd suggest doing the following:

1) Create a src/ directory and move *.c, *.h, *.sh, *.perl, *.py and 
   the builtin directory from the root directory to it.

2) Create a build/ directory, or bin/ if prefered, to hold the result of 
   the build.

3) Consider dropping the ppc/ directory.  Unless someone really cares 
   deeply, it would be nice to simply always use the block-sha1 code and 
   move it straight into src/.

4) Consider moving some more directories into src/ such as xdiff/.
   I'd leave compat/ outside src/ to make it more explicit that this is 
   not about Git proper.

5) Rename t/ to testsuite/ so this doesn't look like some garbage 
   leftover.

6) And fix up all the Makefiles to cope with the above movements.

What do you think?

Nicolas