Sane refresh logic for #1180

Libgit2 for AmigaOS4

All the ODB backends have a specific refresh interface. When reading an
object, first we attempt every single backend: if the read fails, then
we refresh all the backends and retry the read one more time to see if
the object has appeared.

Don't call pthread_exit() in the callback of the fetch example

Compilers that are not aware that pthread_exit() does not return
issue a warning when compiling the present code. This change
exchanges the call to pthread_exit() with a simple return
statement. According to the pthread specification this is
equivalent.

Before it was compiled when CMake was actually run on AmigaOS.

push: properly handle tags

Currently, push doesn't really handle tags when queueing objects. Fix
it.

Resolve crash with diff against empty file

It is not legal inside our `p_mmap` function to mmap a zero length
file.  This adds a test that exercises that case inside diff and
fixes the code path where we would try to do that.

The fix turns out not to be a lot of code since our default file
content is already initialized to "" which works in this case.

Fixes #1210

update copyrights

remove ppc sha1 asm

keep comments at < 80 chars

Have diff blob to buffer share code (and add tests)

This moves the implementation of these two APIs into common code
that will be shared between the two.  Also, this adds tests for
the `git_diff_blob_to_buffer` API.  Lastly, this adds some extra
`const` to a few places that can use it.

Revert changes from git/git diff-delta.c by dak@gnu.org, proski@gnu.org

Add Florian Forster to hall of fame

Permission recieved from Florian Forster on Jan 07 2013 to include
any changes of his from core Git into LibGit2.

merge cleanup should actually cleanup and the test should actually test

This reverts commit 47fbcbb5.

travis: Only run Valgrind when all tests pass

Allow to clone repositories that don't have a `master` branch

travis: Include the online suite when running against Travis