t: avoid alternation (not POSIX) in grep's BRE

f1e3df3 (t: increase test coverage of signature verification output,
2020-03-04) adds GPG dependent tests to t4202 and t6200 that were found
problematic with at least OpenBSD 6.7.

Using an escaped '|' for alternations works only in some implementations
of grep (e.g. GNU and busybox).

It is not part of POSIX[1] and not supported by some BSD, macOS, and
possibly other POSIX compatible implementations.

Use `grep -E`, and write it using extended regular expression.

[1] https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html#tag_09_03

Helped-by: Torsten Bögershausen <[email protected]>
Signed-off-by: Carlo Marcelo Arenas Belón <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: carenas

t/t4202-log.sh

@@ -1643,7 +1643,7 @@ test_expect_success GPG 'log --graph --show-signature for merged tag with missin
 	GNUPGHOME=. git log --graph --show-signature -n1 plain-nokey >actual &&
 	grep "^|\\\  merged tag" actual &&
 	grep "^| | gpg: Signature made" actual &&
-	grep "^| | gpg: Can'"'"'t check signature: \(public key not found\|No public key\)" actual
+	grep -E "^| | gpg: Can'"'"'t check signature: (public key not found|No public key)" actual
 '
 
 test_expect_success GPG 'log --graph --show-signature for merged tag with bad signature' '

t/t6200-fmt-merge-msg.sh

@@ -103,7 +103,7 @@ test_expect_success GPG 'message for merging local tag signed by unknown key' '
 	GNUPGHOME=. git fmt-merge-msg <.git/FETCH_HEAD >actual 2>&1 &&
 	grep "^Merge tag ${apos}signed-good-tag${apos}" actual &&
 	grep "^# gpg: Signature made" actual &&
-	grep "^# gpg: Can${apos}t check signature: \(public key not found\|No public key\)" actual
+	grep -E "^# gpg: Can${apos}t check signature: (public key not found|No public key)" actual
 '
 
 test_expect_success 'message for merging external branch' '