t: convert fgrep usage to "grep -F"

Despite POSIX states that:

> The old egrep and fgrep commands are likely to be supported for many
> years to come as implementation extensions, allowing historical
> applications to operate unmodified.

GNU grep 3.8 started to warn[1]:

> The egrep and fgrep commands, which have been deprecated since
> release 2.5.3 (2007), now warn that they are obsolescent and should
> be replaced by grep -E and grep -F.

Prepare for their removal in the future.

[1]: https://lists.gnu.org/archive/html/info-gnu/2022-09/msg00001.html

Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t9133-git-svn-nested-git-repo.sh b/t/t9133-git-svn-nested-git-repo.sh
index f894860..d8d5362 100755
--- a/t/t9133-git-svn-nested-git-repo.sh
+++ b/t/t9133-git-svn-nested-git-repo.sh
@@ -35,7 +35,7 @@
 		echo b >> a &&
 		svn_cmd commit -m "SVN-side change outside of .git" &&
 		svn_cmd up &&
-		svn_cmd log -v | fgrep "SVN-side change outside of .git"
+		svn_cmd log -v | grep -F "SVN-side change outside of .git"
 	)
 '
 
@@ -59,7 +59,7 @@
 		svn_cmd add --force .git &&
 		svn_cmd commit -m "SVN-side change inside of .git" &&
 		svn_cmd up &&
-		svn_cmd log -v | fgrep "SVN-side change inside of .git"
+		svn_cmd log -v | grep -F "SVN-side change inside of .git"
 	)
 '
 
@@ -82,7 +82,7 @@
 		git commit -m "add a inside an SVN repo" &&
 		svn_cmd commit -m "SVN-side change in and out of .git" &&
 		svn_cmd up &&
-		svn_cmd log -v | fgrep "SVN-side change in and out of .git"
+		svn_cmd log -v | grep -F "SVN-side change in and out of .git"
 	)
 '