diffcore-pickaxe: make error messages more consistent

Currently, diffcore-pickaxe reports two distinct errors for the same
user error:

    $ git log --pickaxe-regex -S'\1'
    fatal: invalid pickaxe regex: Invalid back reference

    $ git log -G'\1'
    fatal: invalid log-grep regex: Invalid back reference

This "log-grep" was only an internal name for the -G feature during
development, and invite confusion with "git log --grep=<pattern>".

Change the error messages to say "invalid regex".

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diffcore-pickaxe.c

@@ -122,7 +122,7 @@ static void diffcore_pickaxe_grep(struct diff_options *o)
 		char errbuf[1024];
 		regerror(err, &regex, errbuf, 1024);
 		regfree(&regex);
-		die("invalid log-grep regex: %s", errbuf);
+		die("invalid regex: %s", errbuf);
 	}
 
 	pickaxe(&diff_queued_diff, o, &regex, NULL, diff_grep);
@@ -246,7 +246,7 @@ static void diffcore_pickaxe_count(struct diff_options *o)
 			char errbuf[1024];
 			regerror(err, &regex, errbuf, 1024);
 			regfree(&regex);
-			die("invalid pickaxe regex: %s", errbuf);
+			die("invalid regex: %s", errbuf);
 		}
 		regexp = &regex;
 	} else {