Introduce die_errno() that appends strerror(errno) to die()

There are many calls to die() that do, or should, report
strerror(errno) to indicate how the syscall they guard failed.
Introduce a small helper function for this case.

Note:

- POSIX says vsnprintf can modify errno in some unlikely cases, so we
  have to use errno early.

- We take some care to pass the original format to die_routine(), in
  case someone wants to call die_errno() with custom format
  characters.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-compat-util.h

@@ -162,6 +162,7 @@ extern char *gitbasename(char *);
 /* General helper functions */
 extern void usage(const char *err) NORETURN;
 extern void die(const char *err, ...) NORETURN __attribute__((format (printf, 1, 2)));
+extern void die_errno(const char *err, ...) NORETURN __attribute__((format (printf, 1, 2)));
 extern int error(const char *err, ...) __attribute__((format (printf, 1, 2)));
 extern void warning(const char *err, ...) __attribute__((format (printf, 1, 2)));
 

usage.c

@@ -60,6 +60,18 @@ void die(const char *err, ...)
 	va_end(params);
 }
 
+void die_errno(const char *fmt, ...)
+{
+	va_list params;
+	char fmt_with_err[1024];
+
+	snprintf(fmt_with_err, sizeof(fmt_with_err), "%s: %s", fmt, strerror(errno));
+
+	va_start(params, fmt);
+	die_routine(fmt_with_err, params);
+	va_end(params);
+}
+
 int error(const char *err, ...)
 {
 	va_list params;