Update to the xdiff used in git v2.35.0, with updates to our build
configuration to ignore the sort of warnings that we normally care
about (signed/unsigned mismatch, unused, etc.)

Any git-specific abstraction bits are now redefined for our use in
`git-xdiff.h`.  It is a (wildly optimistic) hope that we can use that
indirection layer to standardize on a shared xdiff implementation.

COPYING: remove regex copyright, add PCRE copyright

We now bundle PCRE for our regular expression implementation; include
that copyright notice and remove the now-unused regular expression
copyright.

diff_driver: split global_drivers array into separate elements

win32: update git for windows compatibility

C99 doesn't allow structures with flexible length array to be
placed in an array.

Improve documentation

We look for a Git for Windows installation to use its git config,
so that clients built on libgit2 can interoperate with the Git for
Windows CLI (and clients that are built on top of _it_).

Look for `git` both in the `PATH` and in the registry.  Use the _first_
git install in the path, and the first git install in the registry.

Look in both the `etc` dir and the architecture-specific `etc` dirs
(`mingw64/etc` and `mingw32/etc`) beneath the installation root.

Prefer the git in the `PATH` to the git location in the registry so that
users can override that.

Include more tests for this behavior.

Provide a helper function to find an executable in the current process's
PATH.

With Git for Windows >= 2 the share folder is in an architecture specific subfolder.

This also add support for Git for Windows versions between 2 and 2.24 where also the etc folder is in an architecture specific subfolder.

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

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

Removal of the last 4 chars is only required for Git for Windows installations in order to find the "root" folder of the Git installation.

Fixes issue #5127.

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

Could happen if the path to git.exe is near to MAX_PATH and we append a longer subdir such as "share/git-core" to it.

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

Support scp style paths with ports

Fix typos

Instead of trying to figure out a repo's path from a URL by hand, parse
a URL using the parsing functionality.

We occasionally need to determine whether a given string is a URL or
something else.  (The "something else" may be a git path in a different
format, like scp formatting, which needs to be handled differently.)

Provide a mechanism for parsing scp-style paths (eg
`git@github.com:libgit2/libgit2` into the url form
`ssh://git@github.com/libgit2/libgit2`.)

GitHub is removing support for the unauthenticated git protocol; test
with the https protocol.

remote: support `http.followRedirects` (`false` and `initial`) and follow initial redirects by default

c99: change single bit flags to unsigned

Test that we honor `http.followRedirects` when set to initial or false.

Give callers the ability to select how to handle redirects - either
supporting redirects during the initial connection (so that, for
example, `git.example.com/repo` can redirect to `github.com/example/repo`)
or all/no redirects.  This is for compatibility with git.

In redirect application logic, (optionally) allow off-site redirects.

We (thankfully) don't have flaky tests anymore, don't retry them.

The new version of our test git server adds redirect support.  Upgrade
to it.