Attempt authentication when a username is presented but a password is
not; this can happen in particular when users are doing token
authentication and specifying the token in the URL itself.  For example,
`https://token@host/` is a valid URI and should be treated as a username
of `token` with an empty password.

Optional stricter allocation checking (for `malloc(0)` cases)

Co-authored-by: lhchavez <lhchavez@lhchavez.com>

Add `GIT_DEBUG_STRICT_ALLOC` to help identify problematic callers of
allocation code that pass a `0` size to the allocators and then expect a
non-`NULL` return.

When given a 0-size allocation, `malloc` _may_ return either a `NULL`
_or_ a pointer that is not writeable.  Most systems return a non-`NULL`
pointer; AIX is an outlier.  We should be able to cope with this AIXy
behavior, so this adds an option to emulate it.

This refactors this to a wrapper function, as well as changing the
existing API have a va_list and ... version.

stdintification: use int64_t and INT64_C instead of long long

git__timer: Allow compilation on systems without CLOCK_MONOTONIC

For 32-bit int: There's no real reason for that ifdef if we explicitly
specify the stdint.h type.

For 64-bit int: ope, I made it signed accidentally

Makes usage of CLOCK_MONOTONIC conditional and makes functions that uses
git__timer handle clock resynchronization.

Call gettimeofday with tzp set to NULL as required by
https://pubs.opengroup.org/onlinepubs/9699919799/functions/gettimeofday.html

Make `FIND_PACKAGE(PythonInterp)` prefer `python3`

fix check for ignoring of negate rules

Fix wrong time_t used in function

config: fix included configs not refreshed more than once

This change makes it possible to prefer a python (version 3) interpreter
on systems where python2 and python3 coexist (where python2 is found as
`/usr/bin/python`).

Fix memory leak in git_smart__connect

git__timer: Limit ITimer usage to AmigaOS4

Passes w/ gcc 11 on Fedora x64.

Protip: So you don;t have to suffer,

```
perl -pe 's/(-?(?:0x)?[A-Fa-f0-9]+)([Uu])?[Ll][Ll]/\U$2INT64_C(\E$1)/mg'
```

Even on systems without C99 where long long and stdint are both
missing, we can shim stdint and point it to any compiler-specific
type (i.e long long, _int64, etc.).

Also next is constant suffixes and determining what needs to
include stdint.

This function doesn't interoperate with any system functions that
use the system time_t, but rather only works with the git_time_t
type in libgit2, which could be a different width than the system
one. Fixes a compile warning.

Consider files executable only if the user can execute them

Update from regex to pcre licensing information in docs/contributing.md

docs: fix some missing includes that cause Docurium to error out

Define WINHTTP_NO_CLIENT_CERT_CONTEXT if needed

docs: stop mentioning libgit2's "master" branch

Co-authored-by: lhchavez <lhchavez@lhchavez.com>

If an included config is refreshed twice, the second update is not taken
into account.

This is because the list of included files is cleared after re-reading
the new configuration, instead of being cleared before.

Fix it and add a test case to check for this bug.

The name of libgit2's main branch is now "main" but contributing.md and
release.md still reference "master" as the main branch.  Fix it.