0.23 xdiff

xdiff craps the bed on large files.  Treat very large files as binary,
so that it doesn't even have to try.

Refactor our merge binary handling to better match git.git, which
looks for a NUL in the first 8000 bytes.

Maintenance backports for v23

Include the copyright notice from the deps/winhttp/ sources.  Move the
LGPL to the bottom of the file (since multiple dependencies are LGPL
licensed) and include the actual copyright notices from the regex sources.

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.

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.

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

When we're looking to update a tag, we can't stop if the tag auto-follow
rules don't say to update it. The tag might still match the refspec we
were given.

When curl uses a proxy, it will only use Basic unless we prompt it to
try to use the most secure on it has available.

This is something which git did recently, and it seems like a good idea.

OS X may have libssh2 in diff locations, so CHECK_LIBRARY_EXISTS may check the wrong lib; forcing it to use a found directory.

Fix duplicate basenames to support older VS on maint/v0.23

With Visual Studio versions 2008 and older they ignore the full path to files and only check
the basename of the file to find a collision. Additionally, having duplicate basenames can break
other build tools like GYP.

This fixes https://github.com/libgit2/libgit2/issues/3356

A second round of 0.23 maint updates

When we have an error renaming the lockfile, we need to make sure
that we remove it upon cleanup. For this, we need to keep track of
whether we opened the file and whether the rename succeeded.

If we did create the lockfile but the rename did not succeed, we
remove the lockfile. This won't protect against all errors, but
the most common ones (target file is open) does get handled.

When we fail to rename, we currently leave the lockfile laying
around. This shows that behaviour.

For such a path '/~/...' the leading '/' is stripped so the server will
get a path starting with '~' and correctly handle it.

Without this change, compiling with gcc and pedantic generates warning:
ISO C does not allow extra ‘;’ outside of a function.

Fixes issue #3338

We don't want the iterator to make us stop whenever we hit an unreadable
dir. We should instead move over to the next item.

An error here will typically mean that the directory was removed between
the time we iterated the parent and the time we wanted to visit it in
which case we should ignore it.

Other kinds of errors such as permissions (or transient errors) also
better dealt with by pretending we didn't see it.

Maintenance updates for v0.23

(fixes issue #3316)

Signed-off-by: Sven Strickroth <email@cs-ware.de>

git_fetch_options was missing from the API docs because it lacked a
documentation comment above the struct declaration.
I used the git_checkout_options docstring as a template.

Also fixes a typo in git_remote_prune_refs (remote, not reamote).

Our path functions expect to work with slashes, so convert a
path with backslashes into one with slashes at the top of
the function.

The header src/cc-compat.h defines portable format specifiers PRIuZ, PRIdZ, and PRIxZ. The original report highlighted the need to use these specifiers in examples/network/fetch.c.  For this commit, I checked all C source and header files not in deps/ and transitioned to the appropriate format specifier where appropriate.