POSIX-ify the SHELL variable

When calling a non-MSys2 binary, all of the environment is converted from
POSIX to Win32, including the SHELL environment variable. In Git for
Windows, for example, `SHELL=/usr/bin/bash` is converted to
`SHELL=C:\Program Files\Git\usr\bin\bash.exe` when calling the `git.exe`
binary. This is appropriate because non-MSys2 binaries would not handle
POSIX paths correctly.

Under certain circumstances, however, `git.exe` calls an *MSys2* binary in
turn, such as `git config --edit` calling `vim.exe` unless Git is
configured to use another editor specifically.

Now, when this "improved vi" calls shell commands, it uses that $SHELL
variable *without quoting*, resulting in a nasty error:

	C:\Program: No such file or directory

Many other programs behave in the same manner, assuming that $SHELL does
not contain spaces and hence needs no quoting, unfortunately including
some of Git's own scripts.

Therefore let's make sure that $SHELL gets "posified" again when entering
MSys2 programs.

Earlier attempts by Git for Windows contributors claimed that adding
`SHELL` to the `conv_envvars` array does not have the intended effect.
These reports just missed that the `conv_start_chars` array (which makes
the code more performant) needs to be adjusted, too.

Note that we set the `immediate` flag to `true` so that the environment
variable is set immediately by the MSys2 runtime, i.e. not only spawned
processes will see the POSIX-ified `SHELL` variable, but the MSys2 runtime
*itself*, too.

This fixes git-for-windows/git#542,
git-for-windows/git#498, and
git-for-windows/git#468.

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

winsup/cygwin/environ.cc

@@ -326,6 +326,7 @@ 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 ("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},
     {NL ("TEMP="), NULL, NULL, env_path_to_posix, env_path_to_win32, false},
@@ -354,7 +355,7 @@ static const unsigned char conv_start_chars[256] =
     WC,       0,        0,        0,        WC,       0,        0,        0,
     /*  80 */
 /*  P         Q         R         S         T         U         V         W */
-    WC,       0,        0,        0,        WC,       0,        0,        0,
+    WC,       0,        0,        WC,       WC,       0,        0,        0,
     /*  88 */
 /*  x         Y         Z                                                   */
     0,        0,        0,        0,        0,        0,        0,        0,
@@ -405,6 +406,8 @@ win_env::add_cache (const char *in_posix, const char *in_native)
       native = (char *) realloc (native, namelen + 1 + strlen (in_native));
       stpcpy (stpcpy (native, name), in_native);
     }
+  else if (skip_if_empty && !*in_posix)
+    native = (char *) calloc(1, 1);
   else
     {
       tmp_pathbuf tp;
@@ -471,6 +474,8 @@ posify_maybe (char **here, const char *value, char *outenv)
     return;
 
   int len = strcspn (src, "=") + 1;
+  if (conv->skip_if_empty && !src[len])
+    return;
 
   /* Turn all the items from c:<foo>;<bar> into their
      mounted equivalents - if there is one.  */

winsup/cygwin/environ.h

@@ -23,7 +23,7 @@ struct win_env
     char *native;
     ssize_t (*toposix) (const void *, void *, size_t);
     ssize_t (*towin32) (const void *, void *, size_t);
-    bool immediate;
+    bool immediate, skip_if_empty;
     void __reg3 add_cache (const char *in_posix, const char *in_native = NULL);
     const char * get_native () const {return native ? native + namelen : NULL;}
     const char * get_posix () const {return posix ? posix : NULL;}