Starting at OS X 10.8, the Security framework offers some functions
which are unified across OS X and iOS. These are the functions that we
use.

Older versions of OS X do not have these functions and we fail to
compile. In these situations, fall back to using OpenSSL for our TLS
stream instead.

Fix build warning: implicit declaration of function ‘git_transaction_config_new’

Fix bug in git_smart__push: push_transfer_progress cb is only called at end

diff: don't error out on an invalid regex

http: propagate the credentials callback's error code

When parsing user-provided regex patterns for functions, we must not
fail to provide a diff just because a pattern is not well
formed. Ignore it instead.

When we ask for credentials, the user may choose to return EUSER to
indicate that an error has happened on its end and it wants to be given
back control.

We must therefore pass that back to the user instead of mentioning that
it was on_headers_complete() that returned an error code. Since we can,
we return the exact error code from the user (other than PASSTHROUGH)
since it doesn't cost anything, though using other error codes aren't
recommended.

curl: use the most secure auth method for the proxy

The conditional checked cbs->transfer_progress then used the value in cbs->push_transfer_progress. In both cases it should be push_transfer_progress