blame: remove needless comparison with GIT_SHA1_HEXSZ

blame: remove needless comparison with GIT_SHA1_HEXSZ

When faking a working tree commit, we read in lines from MERGE_HEAD into
a strbuf.  Because the strbuf is NUL-terminated and get_oid_hex will
fail if it unexpectedly encounters a NUL, the check for the length of
the line is unnecessary.  There is no optimization benefit from this
case, either, since on failure we call die.  Remove this check, since it
is no longer needed.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>