t: remove \{m,n\} from BRE grep usage

t: remove \{m,n\} from BRE grep usage

The CodingGuidelines says we should avoid \{m,n\} in BRE usage.
And their usages in our code base is limited, and subjectively
hard to read.

Replace them with ERE.

Except for "0\{40\}" which would be changed to "$ZERO_OID",
which is a better value for testing with:
GIT_TEST_DEFAULT_HASH=sha256

Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>