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.

The most prominent user of `git_terminal_prompt()` is certainly
`git-remote-https.exe`. It is an interesting use case because both
`stdin` and `stdout` are redirected when Git calls said executable, yet
it still wants to access the terminal.

When running inside a `mintty`, the terminal is not accessible to the
`git-remote-https.exe` program, though, because it is a MinGW program
and the `mintty` terminal is not backed by a Win32 console.

To solve that problem, we simply call out to the shell -- which is an
*MSys2* program and can therefore access `/dev/tty`.

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

compat/terminal.c

@@ -1,4 +1,4 @@
-#include "git-compat-util.h"
+#include "cache.h"
 #include "compat/terminal.h"
 #include "sigchain.h"
 #include "strbuf.h"
@@ -194,6 +194,54 @@ static int mingw_getchar(void)
 }
 #define getchar mingw_getchar
 
+static char *shell_prompt(const char *prompt, int echo)
+{
+	const char *read_input[] = {
+		/* Note: call 'bash' explicitly, as 'read -s' is bash-specific */
+		"bash", "-c", echo ?
+		"cat >/dev/tty && read -r line </dev/tty && echo \"$line\"" :
+		"cat >/dev/tty && read -r -s line </dev/tty && echo \"$line\" && echo >/dev/tty",
+		NULL
+	};
+	struct child_process child = CHILD_PROCESS_INIT;
+	static struct strbuf buffer = STRBUF_INIT;
+	int prompt_len = strlen(prompt), len = -1, code;
+
+	child.argv = read_input;
+	child.in = -1;
+	child.out = -1;
+
+	if (start_command(&child))
+		return NULL;
+
+	if (write_in_full(child.in, prompt, prompt_len) != prompt_len) {
+		error("could not write to prompt script");
+		close(child.in);
+		goto ret;
+	}
+	close(child.in);
+
+	strbuf_reset(&buffer);
+	len = strbuf_read(&buffer, child.out, 1024);
+	if (len < 0) {
+		error("could not read from prompt script");
+		goto ret;
+	}
+
+	strbuf_strip_suffix(&buffer, "\n");
+	strbuf_strip_suffix(&buffer, "\r");
+
+ret:
+	close(child.out);
+	code = finish_command(&child);
+	if (code) {
+		error("failed to execute prompt script (exit code %d)", code);
+		return NULL;
+	}
+
+	return len < 0 ? NULL : buffer.buf;
+}
+
 #endif
 
 #ifndef FORCE_TEXT
@@ -205,6 +253,12 @@ 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 shell_prompt(prompt, echo);
+#endif
 
 	input_fh = fopen(INPUT_PATH, "r" FORCE_TEXT);
 	if (!input_fh)