mingw: Support git_terminal_prompt with more terminals

The `git_terminal_prompt()` function expects the terminal window to be
attached to a Win32 Console. However, this is not the case with terminal
windows other than `cmd.exe`'s, e.g. with MSys2's own `mintty`.

Non-cmd terminals such as `mintty` still have to have a Win32 Console
to be proper console programs, but have to hide the Win32 Console to
be able to provide more flexibility (such as being resizeable not only
vertically but also horizontally). By writing to that Win32 Console,
`git_terminal_prompt()` manages only to send the prompt to nowhere and
to wait for input from a Console to which the user has no access.

This commit introduces a function specifically to support `mintty` -- or
other terminals that are compatible with MSys2's `/dev/tty` emulation. We
use the `TERM` environment variable as an indicator for that: if the value
starts with "xterm" (such as `mintty`'s "xterm_256color"), we prefer to
let `xterm_prompt()` handle the user interaction.

To handle the case when standard input/output are redirected – as is the
case when pushing via HTTPS: `git-remote-https`' standard input and
output are pipes from/to the main Git executable – we make use of the
`MSYS_TTY_HANDLES` environment variable that was introduced to
fix another bug in MSys2-based Git: this environment variable contains
the Win32 `HANDLE`s of the standard input, output and error as originally
passed from MSys2 to the Git executable, enclosed within space
characters, skipping handles that do not refer to the terminal window
(e.g. when they were redirected). We will only use those handles when
that environment variable lists all three handles because then we can be
100% certain that we are running inside a terminal window, and that we
know exactly which Win32 handles to use to communicate with it.

Helped-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: nalla <nalla@hamal.uberspace.de>

compat/terminal.c

@@ -1,4 +1,6 @@
+#include <inttypes.h>
 #include "git-compat-util.h"
+#include "run-command.h"
 #include "compat/terminal.h"
 #include "sigchain.h"
 #include "strbuf.h"
@@ -91,6 +93,53 @@ static int disable_echo(void)
 	return 0;
 }
 
+static char *xterm_prompt(const char *prompt, int echo)
+{
+	const char *env = getenv("MSYS_TTY_HANDLES");
+	const char *echo_off[] = { "sh", "-c", "stty -echo </dev/tty", NULL };
+	const char *echo_on[] = { "sh", "-c", "stty echo </dev/tty", NULL };
+	static char buffer[1024];
+	DWORD len, dummy;
+	size_t tty0, tty1, tty2;
+	HANDLE in_handle, out_handle;
+
+	if (!env || 3 != sscanf(env,
+	    " %" SCNuPTR " %" SCNuPTR " %" SCNuPTR " ",
+	    &tty0, &tty1, &tty2)) {
+		warning("Cannot read from xterm");
+		return NULL;
+	}
+
+	in_handle = (HANDLE)tty0;
+	out_handle = (HANDLE)tty1;
+
+	if (!echo && run_command_v_opt(echo_off, 0))
+		warning("Could not disable echo on xterm");
+
+	if (!WriteFile(out_handle, prompt, strlen(prompt), &dummy, NULL)) {
+		warning("Could not write to xterm");
+		return NULL;
+	}
+
+	if (!ReadFile(in_handle, buffer, 1024, &len, NULL)) {
+		warning("Could not read from xterm");
+		return NULL;
+	}
+
+	if (len && buffer[len - 1] == '\n')
+		buffer[--len] = '\0';
+	if (len && buffer[len - 1] == '\r')
+		buffer[--len] = '\0';
+
+	if (!echo) {
+		if(run_command_v_opt(echo_on, 0))
+			warning("Could not re-enable echo on xterm");
+		WriteFile(out_handle, "\n", 1, &dummy, NULL);
+	}
+
+	return len == 0 ? NULL : buffer;
+}
+
 #endif
 
 #ifndef FORCE_TEXT
@@ -102,6 +151,12 @@ char *git_terminal_prompt(const char *prompt, int echo)
 	static struct strbuf buf = STRBUF_INIT;
 	int r;
 	FILE *input_fh, *output_fh;
+#ifdef GIT_WINDOWS_NATIVE
+	const char *term = getenv("TERM");
+
+	if (term && starts_with(term, "xterm"))
+		return xterm_prompt(prompt, echo);
+#endif
 
 	input_fh = fopen(INPUT_PATH, "r" FORCE_TEXT);
 	if (!input_fh)