Introduce `git_rebase` to set up a rebase session that can
then be continued.  Immediately, only merge-type rebase is
supported.

Clean up some warnings

config: remove the refresh function and backend field

remote: accept a repo and name for renaming

Fixed memory leak in git_tag_delete()

clar: use a custom temp directory name

[RFC] Use CommonCrypto for hashing

remote: delete git_remote_supported_url()

Remote objects are not meant to be changed from under the user. We did
this in rename, but only the name and left the refspecs, such that a
save would save the wrong refspecs (and a fetch and anything else would
use the wrong refspecs).

Instead, let's simply take a name and not change any loaded remote from
under the user.

This function does not in fact tell us anything, as almost anything with
a colon in it is a valid rsync-style SSH path; it can not tell us that
we do not support ftp or afp or similar as those are still valid SSH
paths and we do support that.

OSX has its own cryptographic library, let's make use of it instead of
calling out to OpenSSL.

We have been refreshing on read and write for a while now, so
git_config_refresh() is at best a no-op, and might just end up wasting
cycles.

Refresh git configuration before looking for the tracking branch redux.

ssl: dump the SSL ciphers in favour of TLS

All versions of SSL are considered deprecated now, so let's ask OpenSSl
to only use TLSv1. We still ask it to load those ciphers for
compatibility with servers which want to use an older hello but will use
TLS for encryption.

For good measure we also disable compression, which can be exploitable,
if the OpenSSL version supports it.

describe: add example

Remote paths: canonicalize UNC paths on Win32

Git for Windows will handle UNC paths only when in forward-slash
format, eg "//server/path".  When given a UNC path as a remote,
rewrite standard format ("\\server\path") into this ridiculous
format.

tree-cache: correct the entry_count calculation

The entry_count field is the amount of index entries covered by a
particular cache entry, that is how many files are there (recursively)
under a particular directory.

The current code that attemps to do this is severely defincient and is
trying to count the amount of children, which always comes up to zero.

We don't even need to recount, since we have the information during the
cache creation. We can take that number and keep it, as we only ever
invalidate or replace.