Properly display error messages during startup

As reported by John Keeping, running "tig blame master" results in:

    user@host ~/src/tig $ tig blame master
    user@host ~/src/tig $d options to blame

    tig 2.0.3 (Dec 20 2014)

    Usage: tig        [options] [revs] [--] [paths]
       or: tig log    [options] [revs] [--] [paths]
       or: tig show   [options] [revs] [--] [paths]
       or: tig blame  [options] [rev] [--] path
       or: tig grep   [options] [pattern]
       or: tig refs
       or: tig stash
       or: tig status
       or: tig <      [git command output]

    Options:
      +<number>       Select line <number> in the first view
      -v, --version   Show version and exit
      -h, --help      Show help message and exit

with the cursor positioned on the second prompt above most of the error
output.

The problem is that done_display() and therefore endwin() is called
twice when die() is called. First via die_callback (which always points
to done_display() once init_display() has been called), and again by the
atexit() handler also installed once init_display() has been called.
atexit() must be there so that ncurses is "shutdown" when Tig exits due
to a signal. The fix therefore checks the "cursed" flag before calling
endwin().

Fixes #385

NEWS.adoc

@@ -46,6 +46,7 @@ Bug fixes:
  - Fix segfault when updating changes in a maximized stage view opened via the
    main view. (GH #376)
  - Handle line number configs where the interval is not specified. (GH #378)
+ - Fix display of error messages during startup. (GH #385)
 
 tig-2.0.3
 ---------

src/display.c

@@ -411,7 +411,9 @@ report(const char *msg, ...)
 static void
 done_display(void)
 {
-	endwin();
+	if (cursed)
+		endwin();
+	cursed = FALSE;
 }
 
 void