Win32: Unicode environment (incoming)

Convert environment from UTF-16 to UTF-8 on startup.

No changes to getenv() are necessary, as the MSVCRT version is implemented
on top of char **environ.

However, putenv / _wputenv from MSVCRT no longer work, for two reasons:
1. they try to keep environ, _wenviron and the Win32 process environment
in sync, using the default system encoding instead of UTF-8 to convert
between charsets
2. msysgit and MSVCRT use different allocators, memory allocated in git
cannot be freed by the CRT and vice versa

Implement mingw_putenv using the env_setenv helper function from the
environment merge code.

Note that in case of memory allocation failure, putenv now dies with error
message (due to xrealloc) instead of failing with ENOMEM. As git assumes
setenv / putenv to always succeed, this prevents it from continuing with
incorrect settings.

Signed-off-by: Karsten Blees <blees@dcon.de>
Signed-off-by: Stepan Kasal <kasal@ucw.cz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/mingw.c

@@ -1260,6 +1260,12 @@ char *mingw_getenv(const char *name)
 	return result;
 }
 
+int mingw_putenv(const char *namevalue)
+{
+	environ = env_setenv(environ, namevalue);
+	return 0;
+}
+
 /*
  * Note, this isn't a complete replacement for getaddrinfo. It assumes
  * that service contains a numerical port, or that it is null. It
@@ -2052,6 +2058,11 @@ void mingw_startup()
 	maxlen = wcslen(_wpgmptr);
 	for (i = 1; i < argc; i++)
 		maxlen = max(maxlen, wcslen(wargv[i]));
+	for (i = 0; wenv[i]; i++)
+		maxlen = max(maxlen, wcslen(wenv[i]));
+
+	/* nedmalloc can't free CRT memory, allocate resizable environment list */
+	environ = xcalloc(i + 1, sizeof(char*));
 
 	/* allocate buffer (wchar_t encodes to max 3 UTF-8 bytes) */
 	maxlen = 3 * maxlen + 1;
@@ -2064,6 +2075,10 @@ void mingw_startup()
 		len = xwcstoutf(buffer, wargv[i], maxlen);
 		__argv[i] = xmemdupz(buffer, len);
 	}
+	for (i = 0; wenv[i]; i++) {
+		len = xwcstoutf(buffer, wenv[i], maxlen);
+		environ[i] = xmemdupz(buffer, len);
+	}
 	free(buffer);
 
 	/* initialize critical section for waitpid pinfo_t list */

compat/mingw.h

@@ -207,6 +207,8 @@ char *mingw_getcwd(char *pointer, int len);
 
 char *mingw_getenv(const char *name);
 #define getenv mingw_getenv
+int mingw_putenv(const char *namevalue);
+#define putenv mingw_putenv
 
 int mingw_gethostname(char *host, int namelen);
 #define gethostname mingw_gethostname