Handle ORIGINAL_PATH just like PATH

MSYS2 recently introduced that hack where the ORIGINAL_PATH variable is
set to the original PATH value in /etc/profile, unless previously set.
In Git for Windows' default mode, that ORIGINAL_PATH value is the used
to define the PATH variable explicitly.

So far so good.

The problem: when calling from inside an MSYS2 process (such as Bash) a
MINGW executable (such as git.exe) that then calls another MSYS2
executable (such as bash.exe), that latter call will try to re-convert
ORIGINAL_PATH after the previous call converted ORIGINAL_PATH from POSIX
to Windows paths. And this conversion may very well fail, e.g. when the
path list contains mixed semicolons and colons.

So let's just *force* the MSYS2 runtime to handle ORIGINAL_PATH in the
same way as the PATH variable (which conversion works, as we know).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

winsup/cygwin/environ.cc

@@ -323,6 +323,7 @@ static win_env conv_envvars[] =
     {NL ("HOME="), NULL, NULL, env_path_to_posix, env_path_to_win32, false},
     {NL ("LD_LIBRARY_PATH="), NULL, NULL,
 			       env_plist_to_posix, env_plist_to_win32, true},
+    {NL ("ORIGINAL_PATH="), NULL, NULL, env_PATH_to_posix, env_plist_to_win32, true},
     {NL ("SHELL="), NULL, NULL, env_path_to_posix, env_path_to_win32, true, true},
     {NL ("TMPDIR="), NULL, NULL, env_path_to_posix, env_path_to_win32, false},
     {NL ("TMP="), NULL, NULL, env_path_to_posix, env_path_to_win32, false},
@@ -349,7 +350,7 @@ static const unsigned char conv_start_chars[256] =
     0,        0,        0,        0,        0,        0,        0,        0,
     /*  72 */
 /*  H         I         J         K         L         M         N         O */
-    WC,       0,        0,        0,        WC,       0,        0,        0,
+    WC,       0,        0,        0,        WC,       0,        0,        WC,
     /*  80 */
 /*  P         Q         R         S         T         U         V         W */
     WC,       0,        0,        WC,       WC,       0,        0,        0,