grep: improve errors for unmatched ( and )

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>