Add a simple getpass() for MinGW

This should be replaced with a graphical getpass() at some stage.

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

compat/mingw.c

@@ -1157,3 +1157,18 @@ 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);
+}