t0021: use Windows-friendly `pwd`

In Git for Windows, when passing paths from shell scripts to regular
Win32 executables, thanks to the MSYS2 runtime a somewhat magic path
conversion happens that lets the shell script think that there is a file
at `/git/Makefile` and the Win32 process it spawned thinks that the
shell script said `C:/git-sdk-64/git/Makefile` instead.

This conversion is documented in detail over here:
https://www.msys2.org/docs/filesystem-paths/#automatic-unix-windows-path-conversion

As all automatic conversions, there are gaps. For example, to avoid
mistaking command-line options like `/LOG=log.txt` (which are quite
common in the Windows world) from being mistaken for a Unix-style
absolute path, the MSYS2 runtime specifically exempts arguments
containing a `=` character from that conversion.

We are about to change `test_cmp` to use `git diff --no-index`, which
involves spawning precisely such a Win32 process.

In combination, this would cause a failure in `t0021-conversion.sh`
where we pass an absolute path containing an equal character to the
`test_cmp` function.

Seeing as the Unix tools like `cp` and `diff` that are used by Git's
test suite in the Git for Windows SDK (thanks to the MSYS2 project)
understand both Unix-style as well as Windows-style paths, we can stave
off this problem by simply switching to Windows-style paths and
side-stepping the need for any automatic path conversion.

Note: The `PATH` variable is obviously special, as it is colon-separated
in the MSYS2 Bash used by Git for Windows, and therefore _cannot_
contain absolute Windows-style paths, lest the colon after the drive
letter is mistaken for a path separator. Therefore, we need to be
careful to keep the Unix-style when modifying the `PATH` variable.

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

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