mingw: allow `git.exe` to be used instead of the "Git wrapper"

Git for Windows wants to add `git.exe` to the users' `PATH`, without
cluttering the latter with unnecessary executables such as `wish.exe`.
To that end, it invented the concept of its "Git wrapper", i.e. a tiny
executable located in `C:\Program Files\Git\cmd\git.exe` (originally a
CMD script) whose sole purpose is to set up a couple of environment
variables and then spawn the _actual_ `git.exe` (which nowadays lives in
`C:\Program Files\Git\mingw64\bin\git.exe` for 64-bit, and the obvious
equivalent for 32-bit installations).

Currently, the following environment variables are set unless already
initialized:

- `MSYSTEM`, to make sure that the MSYS2 Bash and the MSYS2 Perl
  interpreter behave as expected, and

- `PLINK_PROTOCOL`, to force PuTTY's `plink.exe` to use the SSH
  protocol instead of Telnet,

- `PATH`, to make sure that the `bin` folder in the user's home
  directory, as well as the `/mingw64/bin` and the `/usr/bin`
  directories are included. The trick here is that the `/mingw64/bin/`
  and `/usr/bin/` directories are relative to the top-level installation
  directory of Git for Windows (which the included Bash interprets as
  `/`, i.e. as the MSYS pseudo root directory).

Using the absence of `MSYSTEM` as a tell-tale, we can detect in
`git.exe` whether these environment variables have been initialized
properly. Therefore we can call `C:\Program Files\Git\mingw64\bin\git`
in-place after this change, without having to call Git through the Git
wrapper.

Obviously, above-mentioned directories must be _prepended_ to the `PATH`
variable, otherwise we risk picking up executables from unrelated Git
installations. We do that by constructing the new `PATH` value from
scratch, appending `$HOME/bin` (if `HOME` is set), then the MSYS2 system
directories, and then appending the original `PATH`.

Side note: this modification of the `PATH` variable is independent of
the modification necessary to reach the executables and scripts in
`/mingw64/libexec/git-core/`, i.e. the `GIT_EXEC_PATH`. That
modification is still performed by Git, elsewhere, long after making the
changes described above.

While we _still_ cannot simply hard-link `mingw64\bin\git.exe` to `cmd`
(because the former depends on a couple of `.dll` files that are only in
`mingw64\bin`, i.e. calling `...\cmd\git.exe` would fail to load due to
missing dependencies), at least we can now avoid that extra process of
running the Git wrapper (which then has to wait for the spawned
`git.exe` to finish) by calling `...\mingw64\bin\git.exe` directly, via
its absolute path.

Testing this is in Git's test suite tricky: we set up a "new" MSYS
pseudo-root and copy the `git.exe` file into the appropriate location,
then verify that `MSYSTEM` is set properly, and also that the `PATH` is
modified so that scripts can be found in `$HOME/bin`, `/mingw64/bin/`
and `/usr/bin/`.

This addresses https://github.com/git-for-windows/git/issues/2283

Note: This keeps the same, hard-coded MSYSTEM platform support for CMake
as before, and introduces an `msystem' and `mingw-prefix` knob for Meson
(read: neither CMake nor Meson will automatically inherit the setting
from the current build environment).

Helped-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>