path-conversion: Introduce ability to switch off conversion.

When calling windows native apps from MSYS2, the runtime tries to
convert commandline arguments by a specific set of rules. This idea was
inherited from the MSys/MinGW project (which is now seemingly stale, yet
must be credited with championing this useful feature, see MinGW wiki
https://web.archive.org/web/20201112005258/http://www.mingw.org/wiki/Posix_path_conversion).

If the user does not want that behavior on a big scale, e.g. inside a
Bash script, with the changes introduced in this commit, the user can
now set the the environment variable `MSYS_NO_PATHCONV` when calling
native windows commands.

This is a feature that has been introduced in Git for Windows via
git-for-windows/msys2-runtime#11 and it predates
support for the `MSYS2_ENV_CONV_EXCL` and `MSYS2_ARG_CONV_EXCL`
environment variables in the MSYS2 runtime; Many users find the
simplicity of `MSYS_NO_PATHCONV` appealing.

So let's teach MSYS2 proper this simple trick that still allows using
the sophisticated `MSYS2_*_CONV_EXCL` facilities but also offers a
convenient catch-all "just don't convert anything" knob.

Signed-off-by: 마누엘 <nalla@hamal.uberspace.de>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

winsup/cygwin/msys2_path_conv.cc

@@ -341,6 +341,16 @@ path_type find_path_start_and_type(const char** src, int recurse, const char* en
 
     if (*it == '\0' || it == end) return NONE;
 
+    /*
+     * Skip path mangling when environment indicates it.
+     */
+    const char *no_pathconv = getenv ("MSYS_NO_PATHCONV");
+
+    if (no_pathconv) {
+        *src = end;
+        return NONE;
+    }
+
     /* Let's not convert ~/.file to ~C:\msys64\.file */
     if (*it == '~') {
 skip_p2w: