grep: improve errors for unmatched ( and )

Imagine you want to grep for (. Easy:

  $ git grep '('
  fatal: unmatched parenthesis

uhoh. This is plainly wrong. Unless you know specifically that

 (a) git grep has expression groups and '(' ... ')' are used for them.
 (b) you can use -e '(' to explicitly say '(' is what you are looking
     for, not the beginning of a group.

Similarly,

  $ git grep ')'
  fatal: incomplete pattern expression: )

is somehow worse. ")" is a complete regular expression pattern.
Of course, the error wants to say "group" here.
In this case it is also not "incomplete", it is unmatched.

Make them say

  $ ./git grep '('
  fatal: unmatched ( for expression group
  $ ./git grep ')'
  fatal: incomplete pattern expression group: )

which are clearer in indicating that it is not the expression that
is wrong (since no pattern had been parsed at all), but rather that
it is been misconstrued as a grouping operator.

Link: https://bugs.debian.org/1051205
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: nabijaczleweli

grep.c

@@ -621,7 +621,7 @@ static struct grep_expr *compile_pattern_atom(struct grep_pat **list)
 		*list = p->next;
 		x = compile_pattern_or(list);
 		if (!*list || (*list)->token != GREP_CLOSE_PAREN)
-			die("unmatched parenthesis");
+			die("unmatched ( for expression group");
 		*list = (*list)->next;
 		return x;
 	default:
@@ -792,7 +792,7 @@ void compile_grep_patterns(struct grep_opt *opt)
 	if (p)
 		opt->pattern_expression = compile_pattern_expr(&p);
 	if (p)
-		die("incomplete pattern expression: %s", p->pattern);
+		die("incomplete pattern expression group: %s", p->pattern);
 
 	if (opt->no_body_match && opt->pattern_expression)
 		opt->pattern_expression = grep_not_expr(opt->pattern_expression);