patch 9.1.0537: signed number detection for CTRL-X/A can be improved

Problem:  signed number detection for CTRL-X/A can be improved
          (Chris Patuzzo)
Solution: Add the new "blank" value for the 'nrformat' setting. This
          will make Vim assume a signed number only if there is a blank
          in front of the sign.
          (distobs)

fixes: #15033
closes: #15110

Signed-off-by: distobs <cuppotatocake@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/optionstr.c b/src/optionstr.c
index 170d48e..9adb77d 100644
--- a/src/optionstr.c
+++ b/src/optionstr.c
@@ -33,7 +33,7 @@
 static char *(p_dip_values[]) = {"filler", "context:", "iblank", "icase", "iwhite", "iwhiteall", "iwhiteeol", "horizontal", "vertical", "closeoff", "hiddenoff", "foldcolumn:", "followwrap", "internal", "indent-heuristic", "algorithm:", NULL};
 static char *(p_dip_algorithm_values[]) = {"myers", "minimal", "patience", "histogram", NULL};
 #endif
-static char *(p_nf_values[]) = {"bin", "octal", "hex", "alpha", "unsigned", NULL};
+static char *(p_nf_values[]) = {"bin", "octal", "hex", "alpha", "unsigned", "blank", NULL};
 static char *(p_ff_values[]) = {FF_UNIX, FF_DOS, FF_MAC, NULL};
 #ifdef FEAT_CLIPBOARD
 // Note: Keep this in sync with did_set_clipboard()