tests: fix and add lint for non-portable grep --file

The --file option to grep isn't in POSIX[1], but -f is[1]. Let's check
for that in the future, and fix the portability regression in
f237c8b ("commit-graph: implement git-commit-graph write",
2018-04-02) that broke e.g. AIX.

1. http://pubs.opengroup.org/onlinepubs/009695399/utilities/grep.html

Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: avar

t/check-non-portable-shell.pl

@@ -43,6 +43,7 @@ sub err {
 	/\bwc -l.*"\s*=/ and err '`"$(wc -l)"` is not portable (use test_line_count)';
 	/\bhead\s+-c\b/ and err 'head -c is not portable (use test_copy_bytes BYTES <file >out)';
 	/(?:\$\(seq|^\s*seq\b)/ and err 'seq is not portable (use test_seq)';
+	/\bgrep\b.*--file\b/ and err 'grep --file FILE is not portable (use grep -f FILE)';
 	/\bexport\s+[A-Za-z0-9_]*=/ and err '"export FOO=bar" is not portable (use FOO=bar && export FOO)';
 	/^\s*([A-Z0-9_]+=(\w+|(["']).*?\3)\s+)+(\w+)/ and exists($func{$4}) and
 		err '"FOO=bar shell_func" assignment extends beyond "shell_func"';

t/t5318-commit-graph.sh

@@ -134,7 +134,7 @@ test_expect_success 'Add one more commit' '
 	git branch commits/8 &&
 	ls $objdir/pack | grep idx >existing-idx &&
 	git repack &&
-	ls $objdir/pack| grep idx | grep -v --file=existing-idx >new-idx
+	ls $objdir/pack| grep idx | grep -v -f existing-idx >new-idx
 '
 
 # Current graph structure: