add example for git commit
fix example for git add
add example for git push

We avoid abbreviations where possible; rename git_cred to
git_credential.

In addition, we have standardized on a trailing `_t` for enum types,
instead of using "type" in the name.  So `git_credtype_t` has become
`git_credential_t` and its members have become `GIT_CREDENTIAL` instead
of `GIT_CREDTYPE`.

Finally, the source and header files have been renamed to `credential`
instead of `cred`.

Keep previous name and values as deprecated, and include the new header
files from the previous ones.

stash: avoid recomputing tree when committing worktree

Compare buffers in diff example

Implement a new example that resembles the git-stash(1) command.
Right now, it only provides the apply, list, save and pop
subcommands without any options.

This example is mostly used to test libgit2's stashing
performance on big repositories.

Implement a new example that resembles git-config(1). Right now,
this example can both read and set configuration keys, only.