mingw: fix isatty() after dup2()

Since a9b8a09c3c30 (mingw: replace isatty() hack, 2016-12-22), we handle
isatty() by special-casing the stdin/stdout/stderr file descriptors,
caching the return value. However, we missed the case where dup2()
overrides the respective file descriptor.

That poses a problem e.g. where the `show` builtin asks for a pager very
early, the `setup_pager()` function sets the pager depending on the
return value of `isatty()` and then redirects stdout. Subsequently,
`cmd_log_init_finish()` calls `setup_pager()` *again*. What should
happen now is that `isatty()` reports that stdout is *not* a TTY and
consequently stdout should be left alone.

Let's override dup2() to handle this appropriately.

This fixes https://github.com/git-for-windows/git/issues/1077

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

1 files changed, 3 insertions, 0 deletions

diff --git a/compat/mingw.h b/compat/mingw.h
index f31dcff2be1..b04556ce0ef 100644
--- a/compat/mingw.h
+++ b/compat/mingw.h
@@ -390,6 +390,9 @@ int mingw_raise(int sig);
 int winansi_isatty(int fd);
 #define isatty winansi_isatty
 
+int winansi_dup2(int oldfd, int newfd);
+#define dup2 winansi_dup2
+
 void winansi_init(void);
 HANDLE winansi_get_osfhandle(int fd);