usage.c: detect recursion in die routines and bail out immediately

It is theoretically possible for a die handler to get into a state of
infinite recursion.  For example, if a die handler called another function
which itself called die().  Let's at least detect this situation, inform the
user, and call exit.

Signed-off-by: Brandon Casey <bcasey@nvidia.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

usage.c

@@ -6,6 +6,8 @@
 #include "git-compat-util.h"
 #include "cache.h"
 
+static int dying;
+
 void vreportf(const char *prefix, const char *err, va_list params)
 {
 	char msg[4096];
@@ -82,6 +84,12 @@ void NORETURN die(const char *err, ...)
 {
 	va_list params;
 
+	if (dying) {
+		fputs("fatal: recursion detected in die handler\n", stderr);
+		exit(128);
+	}
+	dying = 1;
+
 	va_start(params, err);
 	die_routine(err, params);
 	va_end(params);
@@ -94,6 +102,13 @@ void NORETURN die_errno(const char *fmt, ...)
 	char str_error[256], *err;
 	int i, j;
 
+	if (dying) {
+		fputs("fatal: recursion detected in die_errno handler\n",
+			stderr);
+		exit(128);
+	}
+	dying = 1;
+
 	err = strerror(errno);
 	for (i = j = 0; err[i] && j < sizeof(str_error) - 1; ) {
 		if ((str_error[j++] = err[i++]) != '%')