prompt: respect GIT_TERMINAL_PROMPT to disable terminal prompts

If you run git as part of an automated system, you might
prefer git to die rather than try to issue a prompt on the
terminal (because there would be nobody to see it and
respond, and the process would hang forever).

This usually works out of the box because getpass() (and our
more featureful replacements) will fail when there is no
tty, but this does not cover all cases. For example, a batch
system run via ssh might have a tty, even when the user does
not expect it.

Let's provide an environment variable the user can set to
avoid even trying to touch the tty at all.

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

Documentation/git.txt

@@ -901,6 +901,10 @@ for further details.
 	and read the password from its STDOUT. See also the 'core.askpass'
 	option in linkgit:git-config[1].
 
+'GIT_TERMINAL_PROMPT'::
+	If this environment variable is set to `0`, git will not prompt
+	on the terminal (e.g., when asking for HTTP authentication).
+
 'GIT_CONFIG_NOSYSTEM'::
 	Whether to skip reading settings from the system-wide
 	`$(prefix)/etc/gitconfig` file.  This environment variable can

prompt.c

@@ -58,11 +58,19 @@ char *git_prompt(const char *prompt, int flags)
 			r = do_askpass(askpass, prompt);
 	}
 
-	if (!r)
-		r = git_terminal_prompt(prompt, flags & PROMPT_ECHO);
 	if (!r) {
-		/* prompts already contain ": " at the end */
-		die("could not read %s%s", prompt, strerror(errno));
+		const char *err;
+
+		if (git_env_bool("GIT_TERMINAL_PROMPT", 1)) {
+			r = git_terminal_prompt(prompt, flags & PROMPT_ECHO);
+			err = strerror(errno);
+		} else {
+			err = "terminal prompts disabled";
+		}
+		if (!r) {
+			/* prompts already contain ": " at the end */
+			die("could not read %s%s", prompt, err);
+		}
 	}
 	return r;
 }