move git_getpass to its own source file

This is currently in connect.c, but really has nothing to
do with the git protocol itself. Let's make a new source
file all about prompting the user, which will make it
cleaner to refactor.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

prompt.c

@@ -0,0 +1,48 @@
+#include "cache.h"
+#include "run-command.h"
+#include "strbuf.h"
+#include "prompt.h"
+
+char *git_getpass(const char *prompt)
+{
+	const char *askpass;
+	struct child_process pass;
+	const char *args[3];
+	static struct strbuf buffer = STRBUF_INIT;
+
+	askpass = getenv("GIT_ASKPASS");
+	if (!askpass)
+		askpass = askpass_program;
+	if (!askpass)
+		askpass = getenv("SSH_ASKPASS");
+	if (!askpass || !(*askpass)) {
+		char *result = getpass(prompt);
+		if (!result)
+			die_errno("Could not read password");
+		return result;
+	}
+
+	args[0] = askpass;
+	args[1]	= prompt;
+	args[2] = NULL;
+
+	memset(&pass, 0, sizeof(pass));
+	pass.argv = args;
+	pass.out = -1;
+
+	if (start_command(&pass))
+		exit(1);
+
+	strbuf_reset(&buffer);
+	if (strbuf_read(&buffer, pass.out, 20) < 0)
+		die("failed to read password from %s\n", askpass);
+
+	close(pass.out);
+
+	if (finish_command(&pass))
+		exit(1);
+
+	strbuf_setlen(&buffer, strcspn(buffer.buf, "\r\n"));
+
+	return buffer.buf;
+}