MinGW: Add a simple getpass()

We need getpass() to activate curl on MinGW.  Although the default
Makefile currently has 'NO_CURL = YesPlease', msysgit releases do
provide curl support, so getpass() is used.

[spr: - edited commit message.
      - squashed commit that provides getpass() declaration.]

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/mingw.c

@@ -1156,3 +1156,18 @@ int link(const char *oldpath, const char *newpath)
 	}
 	return 0;
 }
+
+char *getpass(const char *prompt)
+{
+	struct strbuf buf = STRBUF_INIT;
+
+	fputs(prompt, stderr);
+	for (;;) {
+		char c = _getch();
+		if (c == '\r' || c == '\n')
+			break;
+		strbuf_addch(&buf, c);
+	}
+	fputs("\n", stderr);
+	return strbuf_detach(&buf, NULL);
+}

compat/mingw.h

@@ -38,6 +38,8 @@ struct passwd {
 	char *pw_dir;
 };
 
+extern char *getpass(const char *prompt);
+
 struct pollfd {
 	int fd;           /* file descriptor */
 	short events;     /* requested events */