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 allows a convenient catch-all
"just don't convert anything" knob.

Signed-off-by: 마누엘 <nalla@hamal.uberspace.de>

winsup/cygwin/msys2_path_conv.cc

@@ -348,6 +348,14 @@ path_type find_path_start_and_type(const char** src, int recurse, const char* en
         return NONE;
     }
 
+    /*
+     * Skip path mangling when environment indicates it.
+     */
+    const char *no_pathconv = getenv ("MSYS_NO_PATHCONV");
+
+    if (no_pathconv)
+      goto skip_p2w;
+
     /*
      * Prevent Git's :file.txt and :/message syntax from beeing modified.
      */