mingw (git_terminal_prompt): do fall back to CONIN$/CONOUT$ method

To support Git Bash running in a MinTTY, we use a dirty trick to access
the MSYS2 pseudo terminal: we execute a Bash snippet that accesses
/dev/tty.

The idea was to fall back to writing to/reading from CONOUT$/CONIN$ if
that Bash call failed because Bash was not found.

However, we should fall back even in other error conditions, because we
have not successfully read the user input. Let's make it so.

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

compat/terminal.c

@@ -259,7 +259,7 @@ char *git_terminal_prompt(const char *prompt, int echo)
 
 	/* try shell_prompt first, fall back to CONIN/OUT if bash is missing */
 	char *result = shell_prompt(prompt, echo);
-	if (result || errno != ENOENT)
+	if (result)
 		return result;
 
 #endif