Fix compiler warning (git_off_t cast to size_t).

Use size_t for page size, instead of long. Check result of sysconf.
Use size_t for page offset so no cast to size_t (second arg to p_mmap).
Use mod instead div/mult pair, so no cast to size_t is necessary.
Fix grammar of licensing sentence, add IRC links

Also various formatting, wrapping and capitalization tweaks.
Add instructions on running the tests
Fix test repo dir not deleted after running
tests: fix leak
Merge from 2660
Clean up various compiler warnings
Clean up some memory leaks
Cleanup memory leak in ssh transport
config test: clean up memory leak
clone::local test: isalpha -> git__isalpha
global: clean up openssl_locks on shutdown
submodule test: free submodule
mwindow: clean up pack map at shutdown
revwalk::mergebase test: free memory
iterator: free paths when skipping them
Remove unused warning on non-win32
Merge pull request #2612 from ethomson/warnings

Clean up some warnings
Introduce git_rebase to set up a rebase session

Introduce `git_rebase` to set up a rebase session that can
then be continued.  Immediately, only merge-type rebase is
supported.
git_reset: const the git_signature arg