It's hard to remember whether it's `-DUSE_HTTPS=mbedTLS` or
`-DUSE_HTTPS=mbedtls`. Even worse for things like `builtin` which we may
have been inconsistent about. Allow for case insensitive options.
There were a few oddities around HTTPS provider selection: namely,
`GIT_OPENSSL_DYNAMIC` implied `GIT_OPENSSL`, which made a bit of sense,
until we added FIPS support. In addition, dynamic OpenSSL for _hashes_
and dynamic OpenSSL for HTTPS was conflated in a few places.
Untangle these, and make `GIT_HTTPS_*` the define, for consistency with
other feature provider selection.
For consistency, specify the nanosecond option in the same way as other
options, and identify it as such. Split the detection of platform
support (`FindStatNsec`) and its selection (`SelectNsec`).
The `USE_` prefix for inputs denotes a backend; the `DEBUG_` prefix
denotes a debugging option. Make `DEBUG_LEAK_CHECKER` the name of the
leak checking option.
Selecting regular expression backend should be specified in the same way
as everything else; `USE_REGEX`. Keep `REGEX_BACKEND` as an optional
fallback.
When specifying a separate working directory path, the given repository
path should never have a `.git` directory created beneath it. That
simply doesn't make sense.
As a result, the `GIT_REPOSITORY_INIT_NO_DOTGIT_DIR` now _also_ no
longer makes sense. It would only ever be a sensible option when one
wanted a separate `.git` directory and working directory, otherwise the
git files and working directory files would be comingled. Remove the
option entirely.
Don't build docs on pushes to maint branches; those docs should only be
built _on release_. In addition, be safer about not creating an existing
branch from a tracking branch.
git's hash algorithm is sha1dc, it is not sha1. Per Linus:
> Honestly, git has effectively already moved from SHA1 to SHA1DC.
>
> So the actual known attack and weakness of SHA1 should simply not be
> part of the discussion for the next hash. You can basically say "we're
> _already_ on the second hash, we just picked one that was so
> compatible with SHA1 that nobody even really noticed.
Warn users who try to compile with SHA1 instead of SHA1DC.
Update the library's (API) version number to v1.9.0. Also update the
soname version number to 2.0, since we've had breaking ABI changes
to the library.
For enum values that are constructed, not literal integers, we need to
parse the inner / implicit expression. For example:
```
GIT_DIFF_REVERSE = (1u << 0)
```
